Environmental benefits and farmers’adoption of winter cover crops in the North China Plain

Shufang GUO ,Yitao ZHANG ,Limei ZHAI ,Jian LIU ,Hongyuan WANG and Hongbin LIU

1State Key Laboratory of Efficient Utilization of AridandSemi-aridArable Landin Northern China,Key Laboratory of Nonpoint Source Pollution Control,Ministry of Agriculture and Rural Affairs,Institute of Agricultural Resources and Regional Planning,Chinese Academy of Agricultural Sciences,Beijing 100081(China)

2Institute of Agricultural Environment and Resources,Yunnan Academy of Agricultural Sciences,Kunming 650205(China)

3Shandong Yucheng Agro-ecosystem National Observation Research Station,Key Laboratory of Ecosystem Network Observation andModeling,Institute of Geographic Sciences and Natural Resources Research,Chinese Academy of Sciences,Beijing 100101(China)

4The Norwegian Institute of Bioeconomy Research(NIBIO),Akershus 1431(Norway)

ABSTRACT The introduction of cover crops into monoculture systems to improve soil health has been widely adopted worldwide.However,little is known about the environmental risks and application prospects of different cover crops in spring maize(Zea mays L.)monocultures proposed in the North China Plain.A pot experiment was conducted to evaluate the effects of different winter cover crops on subsequent maize yield,soil fertility,and environmental risks of nitrogen(N)loss,and a questionnaire survey was conducted to examine factors influencing farmers’willingness to adopt cover crops in the North China Plain.Based on the same fertilization regime during the maize growing period,four winter cover crop treatments were set up,including bare fallow,hairy vetch(Vicia villosa Roth.),February orchid(Orychophragmus violaceus),and winter oilseed rape(Brassica campestris L.).The results indicated that winter cover crops significantly increased subsequent maize yield and soil organic carbon,total N,and microbial biomass carbon and N compared with the bare fallow treatment.The incorporation of cover crops led to a negligible increase in nitrous oxide(N2O)emissions and had a very limited effect on ammonia(NH3)emissions.The incorporation of February orchid and winter oilseed rape decreased nitrate leaching compared with the hairy vetch treatment in the maize growing season.The N losses via N2O and NH3 emissions and N leaching accounted for 71%-84%of the N surplus.However,yield increase and environmental benefits were not the main positive factors for farmers to accept cover crops.Financial incentive was rated by 83.9%of farmers as an“extremely important”factor,followed by other costs,when considering winter cover cropping.These results indicate that the environmental benefits depend on the type of cover crop.Maintaining high levels of soil fertility and maize yield,providing sufficient subsidies,and encouraging large-area cultivation of cover crops are critical measures to promote winter cover cropping in the North China Plain.

Key Words: cover crop,N leaching,NH3 emission,N2O emission,spring maize,willingness to accept

INTRODUCTION

Optimizing planting structure(including fallowing)is vital for ensuring food security and reducing irrigation demand,especially for winter wheat(Triticum aestivumL.),which is a major food crop on drylands(Renet al.,2021).The North China Plain provides 51%and 35%of the nation’s wheat and maize(Zea maysL.)yields,respectively(National Bureau of Statistics,2019).Winter wheat is a major water consumer and uses more than 70%of the irrigation water owing to low precipitation during its growing season(Leiet al.,2019).As a result,the consumption of irrigation water by winter wheat has become a threat to groundwater resources,which are scarce and have been heavily depleted in recent years(Zhanget al.,2009).To mitigate water deficits,the government has encouraged farmers to replace the conventional winter wheat-summer maize rotation with a rotation system including spring maize(three crops in two years)or a spring maize monoculture system,and the latter reached 20%in 2018(Menget al.,2012;Abdallaet al.,2019;Zhaoet al.,2021;Wanget al.,2023).However,this adjustment results in undesirable bare fallow seasons,raising environmental concerns(Allettoet al.,2022).

Cover crops are recommended as an essential component in many cropping systems for sustainable agriculture(Weil and Kremen,2007;Lehmanet al.,2012).This is because cover cropping can reduce nitrogen(N)loss(Weil and Kremen,2007;Bowenet al.,2018)while providing soil fertility benefits to the subsequent crops(Sainjuet al.,2008;Abdallaet al.,2019).Indeed,several studies have demonstrated that cover crops can scavenge nutrients from soils and reduce N leaching during the non-growing seasons(Blanco-Canquiet al.,2015;Aronssonet al.,2016).The replacement of bare fallow with cover crops increases N retention in soil,which may lead to changes in soil fertility and nutrient cycling(Srgioet al.,2005;Ambrosanoet al.,2011).After cover crop harvesting,the carbon (C) and N sequestered in the biomass are decomposed into inorganic forms and become available for uptake by the subsequent crops(Olesenet al.,2007).A pot experiment by Xieet al.(2018)also reported that soil total N(TN)was higher under the Chinese milk vetch+urea treatment than under the sole urea treatment.The direct uptake of N by cover crops and/or the immobilization of soil residual N can considerably mitigate nitratelossvialeaching(Ruffoet al.,2004;Tonittoet al.,2006).In contrast,Farneselliet al.(2018)reported that pure vetch increasedleaching when the N demand of crops was low and a large amount of N-rich biomass was readily available.The effect of cover crops on N leaching is also related to the length of the cover crop growing season(Thorup-Kristensenet al.,2003).Additionally,the choice of cover crop can affect the abundance of denitrification genes and eventually change nitrous oxide(N2O)emissions(Bowenet al.,2018),which is mainly related to the addition of N to soil and soil moisture changes caused by cover cropping (Peyrardet al.,2016).The emission of N2O mostly depends on the type of cover crop,such as legume,non-legume,or a mixture of both(Kimet al.,2013).Specifically,legume-rice rotations are able to mitigate N2O emissions,largely due to the replacement of chemical N input with the N fixed by biological processes(Caiet al.,2018).However,previous studies on the effects of cover crops on N loss were mainly conducted during the subsequent crop growing season or focused on a single loss way(Xionget al.,2013;Zhaoet al.,2015;Xiaet al.,2016).Few studies have assessed the effects of different types of cover crops on the N fate and balance in winter cover crop-spring maize systems.

Despite their many benefits,cover crops are generally not widely used by farmers due to additional costs and other reasons.In Shanghai,the government provides a subsidy of 4 200-4 950 CNY ha-1for cover cropping during the fallow period between main crops.In North Carolina,it is not cost but residue incorporation that is viewed as an obstacle to cover cropping (O’Connellet al.,2014).In Central Spain,many factors influence farmers’decision on cover cropping(Sastret al.,2017).Therefore,it is necessary to understand which factors influence farmers’acceptance of cover cropping in the North China Plain.

Recently,large and spatially continuous areas of winter wheat-summer maize rotation in the northern part of the North China Plain have been increasingly replaced by winter bare fallow-spring crop,which is termed the“spring maize planting belt phenomenon”(Huanget al.,2012;Wanget al.,2014).In this study,a pot experiment was conducted to assess the effects of converting winter bare fallow to cover cropping on the subsequent maize yield,soil fertility,and environmental risks of N loss in the North China Plain,and a questionnaire survey was conducted to identify barriers to cover crop adoption.The objectives were:i)to evaluate the effect of cover cropping on subsequent maize yield,ii)to evaluate the effect of cover cropping on soil N and C dynamics,iii)to assess the effect of winter cover crop-spring maize rotation on N balance in comparison to bare fallowmaize rotation,and iv)to develop strategies to promote cover cropping in the North China Plain.

MATERIALS AND METHODS

Experimental site andsoil used

The pot experiment was conducted from October 2016 to September 2017 at the Changping Experimental Station,Chinese Academy of Agricultural Sciences,Beijing,China(40°23′13′′N,116°29′49′′E).The experimental site has a typical sub-humid temperate continental monsoon climate with a warm wet summer and a cold winter.The annual mean temperature and precipitation are 11°C and 630 mm,respectively (Zhanget al.,2015).Approximately 80% of the annual rainfall is distributed from June to October.The soil used for the pot experiment was taken from a field near the experimental station with a winter wheat-summer maize rotation.It is a Haplic Luvisol according to the Food and Agriculture Organization of the United Nations Soil Taxonomy,a representative soil in the North China Plain with a bulk density of 1.32 g cm-3,TN of 1.26 g kg-1,total phosphorus(P)of 0.85 g kg-1,soil organic matter of 21.9 g kg-1,and pH 8.48 in the surface layer(0-20 cm).

Experimental design

Three most common cover crop species in the North China Plain,hairy vetch (Vicia villosaRoth.),February orchid(Orychophragmus violaceus),and winter oilseed rape(Brassica campestrisL.),were chosen for the pot experiment(Zhaoet al.,2011,2013;Yanget al.,2018).Four winter cover crop treatments in three replicates were set up in a completely randomized design:i)bare fallow,ii)hairy vetch,iii)February orchid,and iv)winter oilseed rape.The cover crops were sown in early October 2016 in pots (70 cm length,55 cm width,and 50 cm height)containing 100 kg air-dried soil.The seeding rate of cover crops at 3 g pot-1was consistent with the farmers’conventional practices in the North China Plain.All treatments were managed in the same way without irrigation or fertilization during the cover crop growing season.The cover crops were harvested,cut into pieces,and mixed with the topsoil(0-20 cm)in their respective pots in late April 2017.Two weeks later,maize was sown at four seeds per pot,with a plant spacing of 25 cm and row spacing of 40 cm(Fig.1).Each pot received 14.4 g urea,18.6 g superphosphate,and 3.33 g potassium chloride,equivalent to 150 kg N,33 kg P,and 37 kg potassium(K)per hectare,respectively.Half of the N and all P and K fertilizers were applied at sowing(May 24),and the other half of N was top-dressed on July 15.Maize in all pots was harvested on September 20,and it was rain-fed without irrigation throughout the growing season.

Fig.1 Photo showing the spring maize plants grown in the pot experiment conducted outdoor from Oct.2016 to Sep.2017 in the Changping Experimental Station,Chinese Academy of Agricultural Sciences,Beijing,China.

Measurements of N losses

The emissions of N2O from cover crop-maize systems were monitored using the static chamber-gas chromatography technique as described by Zhouet al.(2017).The sampling time was 9:00-11:00 a.m.and lasted for 30 min at intervals of 10 min.The temperature inside the chamber was measured by a thermometer installed on each chamber lid for each sampling event.Fluxes of N2O were measured twice a week from October 2016 to September 2017,and the sampling frequency increased after fertilization and precipitation events but decreased to once a week in winter.The N2O concentrations of gas samples were immediately analyzed using gas chromatography (GC 2010,Shimadzu,Japan).Detailed methods for measuring and calculating N2O flux rates and total N2O emissions were described by Xionget al.(2002).

Ammonia (NH3) emission flux was measured with a chamber using the continuous airflow enclosure method(Tianet al.,1998).A glass absorption flask filled with 60 mL 0.05 mol L-1H2SO4was used to absorb NH3gas.The air exchange rate was set to 15-20 chamber volumes per minute,according to the volume of the emission headspace.Air was pumped for 2 h from 9:00 a.m.to 11:00 a.m.and pushed to flow through the H2SO4solution for each treatment.The ammoniumconcentration in NH3absorbent was determined by flow injection analysis using an AA3 autoanalyzer(Bran+Luebbe,Germany).Daily NH3emission flux was calculated from the average flux measured each day.The measurements were conducted within two weeks of fertilizer application.The total NH3emission was obtained by linear interpolation of the fluxes determined at different times.

Holes were installed at the bottom of the sides of the pots to collect leachates after each rainfall event using pipes controlled by three-way stopcocks during the experimental period.Total N concentration of each leachate was determined spectrophotometrically after alkaline potassium persulfate oxidation(APHA,1998).Theandconcentrations in the NH3absorbents were determined by flow injection analysis using an AA3 autoanalyzer(Bran+Luebbe,Germany).The N leaching loss(Q,kg N ha-1)was calculated as follows:

where Niis the N concentration of theith leachate sample(mg L-1),Diis the leachate depth due to theith rainfall event(mm),andnis the number of leaching events during the experimental period.

Wet N deposition measurement

Plastic sampling buckets pre-cleaned with distilled water were installed 1 m above the ground at the experimental station for rainwater collection during each rainfall event.For lasting rainfall events,samples were collected at 8:00 p.m.using wet-only samplers.Rainfall depths were recorded,and the rainwater samples were analyzed for TN concentration spectrophotometrically after alkaline potassium persulfate oxidation(APHA,1998).Wet deposition(W,kg N ha-1)was calculated as follows:

where TNiis the TN concentration of theith rainfall event(mg L-1),diis the depth of theith rainfall event(mm),andmis the number of rainfall events during the experimental period.

Soil andplant sampling andanalyses

Topsoil(0-20 cm)samples were collected after maize harvest.All fresh soil samples were homogenized by mixing and passed through a 2-mm sieve.A portion of each sample was stored at 4°C for the determinations of,and microbial biomass C (MBC) and N (MBN).The remaining soil was air-dried for the determinations of TN,soil organic C(SOC),and other properties.Soilandwere extracted using 100 mL 0.01 mol L-1CaCl2with shaking for 30 min,and the extracts were filtered through 0.45-μm filter membranes and analyzed with an AA3 autoanalyzer(Bran+Luebbe,Germany).Soil MBC and MBN were determined as described by Voroneyet al.(2006).Briefly,the soil samples were chloroform-fumigated,extracted with 60 mL 0.5 mol L-1K2SO4for 1 h,filtered,and quantified with a TOC analyzer(Vario TOC Cube,Elementar,Germany).Soil water content was determinedviaoven-drying at 105°C for 24 h.Soil TN was determined using the semi-micro-Kjeldahl method,and SOC was measured by the KMnO4-oxidizable C estimation method(Weilet al.,2003).

Cover crop samples were collected in late April,and maize plant samples(including grains,straw,and stems)were collected after harvest.After washed with tap water,the plant samples were dried at 70°C for 72 h to a constant weight.The TN concentrations of plant samples were analyzed using a Kjeldahl analyzer after digesting the samples with H2SO4-H2O2(Liuet al.,1996).The total N uptake was calculated as the sum of each part and converted to per unit area.

The N surplus and balance were calculated as follows(Zhanget al.,2019):

where N emission includes N2O and NH3emissions.

Survey method

The questionnaire survey was conducted in September 2016 in Beijing,where the area of bare cropland in winter is large and increasing and people have high demands on the natural environment.Six out of 16 counties in Beijing were selected,including Miyun,Huairou,Yanqing,Shunyi,Daxing,and Tongzhou,with the sown areas of grain crops larger than 10 000 ha.In each county,the town with the largest area of arable land was selected,and a total of 15 villages were surveyed.A total of 112 farmers were visited and questioned regarding their knowledge of cover crops and willingness to plant cover crops.Although there were no cover crop growers,some cover crops were planted under greening trees by the government.The survey questionnaire included information about cropping systems,people engaged in agriculture,farmers’ knowledge of cover crops,factors preventing farmers from cover cropping,and factors associated with the expansion of cover cropping.The percentages of farmers at the age of 50-60 and over 60 were 48.2%and 39.3%,respectively,and with an education level below junior school was 88.4%.Generally,1-2 persons in a family were engaged in agricultural production.

Statistical analyses

Statistical analyses were performed using the SPSS 19.0 statistical software,and graphs were plotted using Microsoft Excel 16.0.Data are presented as mean±standard error(n=3).One-way analysis of variance(ANOVA)and Duncan’s test were employed to determine the significance of differences between treatments atP ＜0.05.

RESULTS

Plant grain yieldandN uptake

The maize grain yield ranged from 8.85±0.16 to 10.83±0.17 Mg ha-1in the winter cover crop-maize systems and was significantly higher than that in the bare fallow-maize system (Table I).Moreover,the maize grain yields in the February orchid and winter oilseed rape treatments were significantly higher than that in the hairy vetch treatment.The cover crop treatments increased the N uptake of maize plants,and the grains showed the highest N uptake.

Contents of SOC,TN,MBC,andMBN

Compared with bare fallow,the cover crop treatments significantly increased SOC content in the 0-10 and 10-20 cm soil layers by 2.23%-5.72%and 7.77%-9.49%,respectively (Fig.2).The hairy vetch and winter oilseed rape treatments had significantly higher SOC contents than the February orchid treatment.The cover crop treatments significantly increased soil TN content in the 0-10 cm layer,and the February orchid and winter oilseed rape treatments significantly increased soil TN content in the 10-20 cm layer as well.However,the soil TN content did not differ significantly among the three cover crop treatments.

TABLE ISpring maize grain yield and N uptake at harvest in the different winter cover crop treatments of the pot experiment conducted outdoor from Oct.2016 to Sep.2017 in the Changping Experimental Station,Chinese Academy of Agricultural Sciences,Beijing,China

Fig.2 Soil organic C(SOC)and total N(TN)contents in the 0-10 and 10-20 cm layers at spring maize harvest in the different winter cover crop treatments of the pot experiment conducted outdoor from Oct.2016 to Sep.2017 in the Changping Experimental Station,Chinese Academy of Agricultural Sciences,Beijing,China.Error bars are standard errors of the means(n=3).Different letters above the bars for a same soil layer indicate significant differences between treatments at P ＜0.05.BF=bare fallow;HV=hairy vetch;FO=February orchid;WOR=winter oilseed rape.

Significant differences in MBC and MBN contents between the bare fallow and cover crop treatments were observed on June 8,2017,40 d after the cover crops were harvested and incorporated into the soils(Fig.3).On September 20,2017,when the maize was harvested,the soil MBC and MBN contents in the cover crop treatments were 7.09%-24.9%and 38.2%-59.4%,respectively,higher than those in the bare fallow treatment.Of the cover crop treatments,the highest and lowest MBC contents were found in the hairy vetch(117.2 mg kg-1)and winter oilseed rape treatments(107.4 mg kg-1),respectively,and the lowest MBN content was found in the hairy vetch treatment(16.6 mg kg-1).

Fig.3 Soil microbial biomass C(MBC)and N(MBN)contents on Jun.8,2017(i.e.,40 d after the cover crops were harvested and incorporated into the soils)and Sep.20,2017(i.e.,the day spring maize was harvested)in the different winter cover crop treatments of the pot experiment conducted outdoor from Oct.2016 to Sep.2017 in the Changping Experimental Station,Chinese Academy of Agricultural Sciences,Beijing,China.Error bars are standard errors of the means(n=3).Different letters above the bars for a same date indicate significant differences between treatments at P ＜0.05.BF=bare fallow;HV=hairy vetch;FO=February orchid;WOR=winter oilseed rape.

N losses

An abrupt increase in the cumulative soil N2O emissions occurred after a big rainfall event on June 26(Fig.4).Another peak,although small,was observed on July 15,when fertilization was performed and rainfall occurred.The emissions of N2O during the maize growing season accounted for 59.5%-92.9%(average:81.3%)of the total emissions throughout the cover crop-maize rotation.The cover crop incorporations increased N2O emissions by 13.9%-114%compared to bare fallow,and hairy vetch planting resulted in significantly lower emissions compared to the other two cover crops.

Fig.4 Cumulative N2O and NH3 emissions in the different winter cover crop treatments of the pot experiment conducted outdoor from Oct.2016 to Sep.2017 in the Changping Experimental Station,Chinese Academy of Agricultural Sciences,Beijing,China.The two arrows indicate fertilization events.Error bars are standard errors of the means(n=3).Different letters above the bars within each treatment indicate significant differences between periods at P ＜0.05.BF=bare fallow;HV=hairy vetch;FO=February orchid;WOR=winter oilseed rape.Period I=21 d after basal fertilization(May 25-Jun.14);Period II=24 d after topdressing(Jul.16-Aug.8);Whole span=whole span of maize growing season(May 24-Sep.20).

The emission of NH3was mainly observed after fertilizer application.Compared to the bare fallow treatment,the February orchid treatment increased the cumulative NH3emissions by 68.6%in the maize season,whereas the hairy vetch and the winter oilseed rape treatments reduced NH3emissions by 5.23%-13.1%(P ＞0.05)(Fig.4).However,there were no significant differences between the three cover crop treatments.

The only one leaching event occurred on July 7 during the maize growing season.The cover crop treatments increased TN leaching loss during the maize growing season by 14.0%-154%compared to the bare fallow treatment(Fig.5).However,the winter oilseed rape and February orchid treatments decreasedleaching compared to bare fallow.For the cover crop treatments,the amounts oflossvialeaching were in the order of hairy vetch＞winter oilseed rape＞February orchid.The soilcontents in the cover crop treatments were significantly lower than that in the bare fallow treatment on June 8,before the leaching event on July 7,and decreased sharply after the leaching event.

Fig.5 and total N(TN)leaching losses during the leaching event on Jul.7,2017 and soil contents before(Jun.8,2017)and after the leaching event(Jul.12,2017)during the spring maize growing season in the different winter cover crop treatments of the pot experiment conducted outdoor from Oct.2016 to Sep.2017 in the Changping Experimental Station,Chinese Academy of Agricultural Sciences,Beijing,China.Error bars are standard errors of the means(n=3).Different letters above the bars for a same parameter or date indicate significant differences between treatments at P ＜0.05.BF=bare fallow;HV=hairy vetch;FO=February orchid;WOR=winter oilseed rape.

N balance andN surplus

There were evident seasonal variations in wet N deposition (Fig.6).Monthly deposition ranged from 0.001 to 5.77 kg N ha-1and was significantly positively related to rainfall depth (data not shown).The annual N deposition was 22.4 kg N ha-1,67.8%of which occurred during the maize growing season.Wet and dry N depositions contributed 15.7%of the annual N input to the systems,whereas fertilizer application contributed the remaining percentage(Fig.7).Crop N uptake in grains and straw was the primary N output from the systems.The largest N surplus occurred in the February orchid treatment(54.4 kg N ha-1),followed by the bare fallow treatment(50.5 kg N ha-1).The lowest N surplus was observed in the hairy vetch treatment(39.8 kg N ha-1).

Fig.6 Monthly wet N deposition during the experimental period from Oct.2016 to Sep.2017 in the Changping Experimental Station,Chinese Academy of Agricultural Sciences,Beijing,China.

Fig.7 N inputs,outputs,and balance in the different winter cover crop treatments of the pot experiment conducted outdoor from Oct.2016 to Sep.2017 in the Changping Experimental Station,Chinese Academy of Agricultural Sciences,Beijing,China.The N dry deposition value(5.6 kg N ha-1)was cited from Pan et al.(2012).BF=bare fallow;HV=hairy vetch;FO=February orchid;WOR=winter oilseed rape.

Nitrogen lossviaN2O and NH3emissions contributed 52.1%of the N surplus in the bare fallow treatment and was lower than those from the cover crop treatments.Regarding the annual N balance,the winter oilseed rape and February orchid treatments had negative balances of 1.40 and 16.3 kg N ha-1,respectively,the hairy vetch treatment had a positive balance of 1.09 kg N ha-1,and the bare fallow treatment had a positive balance of 17.9 kg N ha-1.

Farmers’willingness to adopt cover crops

The questionnaire survey results showed that 83.9%of the respondents rated financial incentive,75.0%rated other costs,and 30%-50%rated irrigation cost,fertilization cost,time and energy costs,or equipment cost as the“extremely important”factors when considering cover cropping(Fig.8).Subsidy and unified cultivation by the government were rated as the“1st most needed”facilities/policies by 67.0% and 14.6%of the respondents,respectively,for cover cropping in the North China Plain.Free seeds and subsidy were rated as the“2nd most needed”by 44.8% and 20.8% of the respondents,respectively.Services(such as sowing and mowing)and free seeds were rated as the“3rd most needed”by 47.7%and 25.8%of the respondents,respectively.

Fig.8 Questionnaire survey results showing the respondents’(i.e.,farmers’)perceptions on the importance levels of factors influencing their adoption of winter cover cropping and the top five most needed facilities/policies.1=financial incentives;2=other costs;3=irrigation cost;4=equipment cost;5=time and energy costs;6=fertilization cost;7=effect on the yield of subsequent crop;8=profitability;9=effect on the on-time sowing of subsequent crop;10=relevant policies;11=seed availability;12=easiness of harvest and incorporation;13=farm size;14=techniques and services;15=effect on soil nutrients;16=climate;17=cover cropping benefits;A=free seeds;B=services such as sowing and mowing;C=technology;D=subsidy;E=unified cultivation by the government.

DISCUSSION

Effects of cover crops on soil fertility andmaize grain yield

Cover crops play an important role in increasing SOC stocks,soil fertility,and soil microbial activity of croplands due to the C and N inputs from cover crop residue decomposition (Poeplau and Don,2015;Chahal and Van Eerd,2018).In this study,the cover crop treatments increased SOC and TN contents compared to the bare fallow treatment(Fig.2).The large amount of N in green manure biomass is retained in soil in organic forms during the subsequent crop growing season,along with increasing C input(Mazzonciniet al.,2011).Similarly,Zhenget al.(2018)found that cover crops increased SOC and TN contents as well as soil enzyme activities in an apple orchard owing to the degradation of cover crop residues.A four-year field study indicated that the increase in soil TN was related to N supplementation and the enhanced mineralization of soil residual organic matter by February orchid incorporation,as indicated by the higher MBN in the cover crop treatment compared to the fertilizer treatment (Yanget al.,2018).It is known that MBC and MBN reflect changes in soil quality and nutrient dynamics resulting from management practices,such as cover cropping(Sainjuet al.,2007).The results of the present study indicated that the cover crops significantly improved soil MBC and MBN(Fig.3).Soil microbial biomass is involved in the decomposition of organic materials.Zhuet al.(2012)also found that the introduction of hairy vetch as a winter cover crop improved MBN in a double rice system in southern China.Compared to hairy vetch and February orchid,the incorporation of winter oilseed rape led to a significantly higher SOC(Fig.2),which was due to its higher biomass and C/N ratio.

The short-term cover crop planting also increased maize yield(Table I).Previous studies have shown that crop yield is related to soil quality and increases with increasing SOC content(Sainjuet al.,2003;Chahal and Van Eerd,2018).However,there is a lack of consensus in the literature regarding the effects of cover crops on subsequent crop yield.Miguez and Bollero (2005) showed that winter grass cover crops had no effect on maize yield,with or without N fertilizer use.In intensively fertilized systems,cover crops show little or no benefit to the subsequent crops and may even reduce crop yield in some cases(Tonittoet al.,2006).The N content and C/N ratio of cover crop may influence its effects on the subsequent crop yield(Kuo and Jellum,2002).In addition,water consumption and nutrient immobilization by cover crop may lead to yield reduction of the subsequent crop(Thorup-Kristensenet al.,2003;Bodneret al.,2007).

Effects of cover crops on N losses

Cover crops change soil C and N contents and thus might lead to different N2O and NH3emissions (Olesenet al.,2007).In this study,the cover crop treatments increased N2O emissions compared to the bare fallow treatment(Fig.4).Some studies have shown that cover crops have a marginal effect on N2O emissions,especially when the results are interpreted on a yearly scale (Liebiget al.,2010;Bascheet al.,2014;Sanz-Cobenaet al.,2014;Peyrard,2016).The immobilization of soil mineral N by cover crops may not be sufficient to reduce N2O emissions from fertilized soils(Mitchellet al.,2013).However,the mineralization of C from cover crop residues shortly before N application may stimulate denitrification and N2O emissions(Sarkodie-Addoet al.,2003;Petersenet al.,2011;Prechslet al.,2017).In particular,leguminous cover crops are likely to release mineral N rapidly after being harvested and incorporated into the soil.Therefore,higher N2O emissions are generally measured from soils planted with N-rich leguminous cover crops than with non-legumes and mixtures of legumes and non-legumes(Prechslet al.,2017).However,hairy vetch,a leguminous cover crop,led to lower N2O emissions compared to the other non-leguminous cover crops.The poor growth of hairy vetch in the first year affected the amount of N released from the decomposition of its residues(Peyrardet al.,2016).In addition,the low mineral N in the hairy vetch treatment due to the high N uptake by maize may have contributed to the small N2O emissions(Table I).

The NH3emitted to the atmosphere can be converted to ammonium aerosols and form particulate material with aerodynamic diameter smaller than 2.5 μm(PM2.5),exacerbating haze formation(Fenget al.,2022).The cover crops had no significant effect on NH3emissions(Fig.4).Under the same chemical fertilizer application regime,February orchid planting has no significant effect on NH3emissions during the maize growing season in the North China Plain(Xionget al.,2013).In general,NH3emissions associated with cover crop residues are rather low compared to emissions from animal husbandry and NH3-based fertilizer applications(Beheraet al.,2013).However,Baiet al.(2015)showed that the incorporation of green manure decreased NH3emission compared to traditional practices,mainly because of the reduction in chemical fertilizer application.Of the three cover crop treatments,the lowest NH3emissions were observed in the hairy vetch treatment.This may be related to the high N uptake by hairy vetch due to biological N fixation and low mineral N in the soil.

Nitrate leaching,a major N loss pathway in agriculture,is controlled by biogeochemical or hydrological processes as well as plant-soil processes such as microbial or plant immobilization (Liet al.,2006;Juet al.,2009).The occurrence ofleaching coincided with a decline in soilcontent (Fig.5).This is in line with the findings of Cuiet al.(2014),who reported that the vertical migration offrom the topsoil to deeper soil layers contributed to soilleaching.Cover crop incorporation promotes soil N availability through N mineralization,which leads to high N leaching loss when high precipitation coincides with the rapid mineralization of cover crop residues(Sarrantonio and Scott,1988;Drinkwateret al.,2000).The February orchid and winter oilseed rape treatments decreasedleaching loss in the maize growing season.The leaching ofdecreased when the addition of organic sources,such as cover crops,led to a better match of N supply with crop N demand (Chenget al.,2017;Yaoet al.,2018).The N released quickly from legume residues and not taken up by crop would result in higherleaching compared to bare fallow(Quemadaet al.,2013),which was the case in this study.

Nutrient balance has been employed as an agroenvironmental indicator to assess nutrient management and potential nutrient loss from agricultural systems (Ju and Zhang,2017).The N balance for legume cover crop is always positive due to additional biological N fixation,which provides 60%-70%of the absorbed N(Landrisciniet al.,2019).The N input of the hairy vetch treatment was the highest when the biological N fixation by hairy vetch(approximately 31 kg N ha-1)was taken into account.However,the amount of biologically fixed N requires further study.The biological N fixation by legume cover crops allows less chemical N fertilizer application in the maize growing season,which would be conducive to further reducing N loss.

Incentive measures to promote cover crop cultivation

Excessive N fertilization in the North China Plain has resulted in serious environmental problems(Juet al.,2009;Chenet al.,2017).Compared to winter wheat-summer maize rotation,winter cover crop-spring maize systems reduce water consumption and fertilizer application in the maize growing season,thus decreasing N leaching due to the reduction of soil mineral N(Cuiet al.,2014).The present study evidenced that cover crops can increase soil nutrient contents and bring yield benefits.February orchid and winter oilseed rape planting decreasedleaching.The questionnaire survey results showed that subsidy and unified cultivation by the government were rated as the“1st most needed facility/policy”by the majority of the respondents(Fig.8).Financial incentives,such as cost-share payment,have previously been identified as an important influencing factor of conservation practice adoption because conservation practices often require long-term investment while providing little or no short-term benefits(Ryanet al.,2003;Lichtenberg,2004).For example,the availability of costshare payments is one of the most important determinants of cover crop adoption across the United States corn belt(Singeret al.,2007).Farmers have been subsidized by the Chinese government since 2006 for planting cover crops(Caoet al.,2017).However,in USA,only 8%of the cover crop growers had received a cost-share payment or other financial incentives,and the number one obstacle was the time and labor involved in cover crop planting to improve soil organic matter,reduce soil erosion,and prevent soil compaction(Myers and Watts,2015).Therefore,promoters of cover crops should showcase local examples where obstacles have been successfully overcome and publicize the benefits of cover crops among farmers.

CONCULSIONS

This short-term study revealed that soil fertility,maize yield,and N loss were influenced when bare fallow in spring maize monoculture was converted to cover crop planting in the North China Plain.The winter cover crop treatments increased SOC,TN,MBC,MBN,and maize yield compared to the bare fallow treatment.The leaching ofwas reduced in the February orchid and winter oilseed rape treatments,and NH3emission was not significantly changed in the cover crop treatments.However,N2O emission was increased in the cover crop treatments,and the lowest increase was found in the hairy vetch treatment.These N losses occurred mainly in the maize growing season.The different cover crops displayed different environmental benefits,deserving further research.Financial incentives and related costs would affect farmers’willingness to adopt cover crops.The government can encourage farmers to adopt cover crops in the North China Plain by providing subsidies,unified cultivation,and relevant equipments and services.

ACKNOWLEDGEMENT

This study was supported by the National Key Research and Development Program of China(No.2022YFD 1700700),the Fundamental Research Funds for Central Nonprofit Scientific Institution,China (No.1610132022008),and the Science and Technology Program of Beijing,China(No.D161100005516002).