Changes in soil properties under Eucalyptus relative to Pinus massoniana and natural broadleaved forests in South China

Shuangshuang Chu•Douglass F.Jacobs•Joshua L.Sloan•Li Xue•Daoming Wu•Shucai Zeng

Abstract The ecological effects of eucalypt plantations(EPs)have garnered increasing attention.To understand their effect on soil quality at a landscape scale,and to determine whether soil quality parameters differ due to different stand types,we evaluated soil characteristics in twenty-one groups of EPs,Pinus massoniana Lamb.plantations(PMPs)and natural broadleaved forests(NBFs)across Guangdong Province,China.Both the physical characteristics of soil hydrology and the properties of soil nutrients in A and B horizons were determined.Results showed that,compared to NBFs,EPs and PMPs produced a shallower litter layer,reduced canopy density,higher soil bulk density,significantly lower total porosity,non-capillary porosity,total water volume,and hygroscopic water in the A horizon(P＜0.05).Moreover,total N,available K,and soil organic carbon(SOC)in EPs and PMPs were significantly lower than in NBFs.EPs and PMPs did not differ significantly in N,P or K content,but PMPs had significantly lower SOC and boron in the A horizon than EPs.Low pH and poor capacity to buffer acidification generally occurred in all cover types.Both EPs and PMPs showed a decline in soil properties relative to NBFs,but EPs and PMPs exhibited no significant difference.These results indicate that actions are needed to ameliorate the potential negative effects on soil quality in forestry plantations.

Keywords Eucalyptus·Pinus massoniana·Plantations·Soil organic matter·Soil acidification ·Moisture retention capacity

Introduction

Eucalyptusare one of the most widely-planted genus of trees owing to their favorable properties such as high adaptability to a range of soils,various climates and ease of management(Zhang et al.2015;Cook et al.2016).Moreover,Eucalyptusplantations(EPs)are one of the most productive ecosystems(Laclau et al.2005).To meet an increasing demand for wood products,China has invested heavily in converting millions of hectares of degraded lands,croplands,and natural forests to EPs(Zhang et al.2015).At the end of 2013,there were over 4.46×106ha of EPs in the country,with the majority in Guangxi Province,Guangdong Province,and Hainan Province(State Forestry Administration of the People’s Republic of China 2014).

With the fast expansion of EPs,concerns have been expressed regarding possible ecological impacts especially on soil quality.Soil quality is the capacity to sustain biological productivity,maintain environmental quality,and promote plant and animal health(Doran and Parkin 1994).Some physical,chemical,and biological properties such as bulk density,water content,pH,nutrient content,and respiration are basic indicators of soil quality(Doran and Parkin 1994).Many studies have reported that soil quality is significantly different between EPs and natural forests(Bargali et al.1993;Khanmirzaei et al.2009;Zhang et al.2015;Kerr and Ruwanza 2016).

Several studies assumed that EPs,especially monocultures ofEucalyptusestablished after clear-cutting natural forests,caused declines in soil quality.Bargali et al.(1993)found that various soil properties under EPs,including water-holding capacity,organic matter content and nutrient concentrations were lower than under natural mixed broadleaved forests;moreover,these properties declined further with the age of EPs.Other studies consistent with the results of Bargali et al.have been published(Li 2007;Alem et al.2010;Zhao et al.2014;Zhang et al.2015;Kerr and Ruwanza 2016).Some reports,however,have noted that soil degradation does not develop with consistency among EPs.For example,Khanmirzaei et al.(2009)found that there were no significant changes in soil properties,especially in levels of N,P,or K followingEucalyptusplanting.Lemenih et al.(2004)found that organic carbon,N,P,and pH increased and soil bulk density decreased under EPs when compared with natural forests.According to Liao et al.(2012),these inconsistent results are possibly related to stand types,land-use history,and geographic conditions in the study regions.

The development of organic carbon and acidification under EPs has attracted significant attention in recent years.Soil organic carbon(SOC)contributes to carbon cycling,nutrient retention,water-holding capacity and biological activities in forest soils(Nambiar and Harwood 2014).Soil acidification has been recognized as one of the main contributors to the degradation of forest ecosystems(Huang et al.2015;Yang et al.2015).Several adverse impacts on SOC and pH under EPs have been shown(Bargali et al.1993;Li 2007;Zhao et al.2014;Zhang et al.2015;Cook et al.2016).Li(2007)found that,after more than 40 years of EPs,the soil quality of basalt-derived laterite and coastal deposit-derived laterite was negatively affected.Soils beneath the EPs have been increasingly acidified since the 1950s,with pH decreasing from 4.7 to 4.1 in basalt-derived laterite and from 5.2 to 4.7 in coastal-deposit-derived laterite in the upper 20-cm layers.Moreover,soil organic matter(SOM)decreased by 48.7 and 37.7%respectively,in these soils.

Similar toEucalyptus,Pinus massoniana,a native species endemic to China,is also regarded as an appropriate species to build fast-growing and high-yield plantations(Tao et al.2016).However,it has been reported that monospecificP.massonianaplantations(PMPs)cause soil degradation and plantations of the species have become one of the most degraded ecosystems in southern China(Chen et al.2013;Wang et al.2013;Tao et al.2016).Wang et al.(2013)found that,compared with plantations of mixedP.massonianaandCastanopsis hystrixA.DC.,plantations of monocultureP.massonianahad 14.3%lower SOC in the upper 20-cm layer.Furthermore,Guo et al.(2015)showed that PMPs had lower nutrients than EPs and mixed plantations because of a reduced capacity for the return of litter nutrients.

Most of the last studies,however,were conducted on individual sites covering a limited area,with few reported soil changes under EPs relative to NBFs at a large(e.g.,provincial)scale.The use of an expanded array of sites across a relatively large area with more replicates might bring new and convincing insights to understanding whether soil properties change with EPs.

In the present study,we investigated changes in soil properties including hydrological characteristics,nutrient content,pH,and SOC underEucalyptusplantations(EPs),P.massonianaplantations(PMPs),and natural broadleaved forests(NBFs)by collecting soil samples from twenty-one sites across Guangdong Province.We hypothesized that soil quality parameters of EPs would significantly differ from PMPs and NBFs at a provincial spatial scale.

Materials and methods

Site descriptions

The study sites were located throughout Guangdong Province(20°09′–25°31′N,109°45′–117°20′E).This province has a tropical to subtropical monsoon climate with annual temperatures ranging from 19.6 to 22.2°C and annual precipitation from 1566 to 2133 mm(Huang et al.2015).Forest coverage was 51.3%,and total forest area was over 9.06×107ha by the end of 2013(State Forestry Administration of the People’s Republic of China 2014).Natural broadleaved forests(NBFs),Eucalyptus urophyllaS.T.Blake plantations(EPs)andP.massonianaplantations(PMPs)were selected because they are all widely distributed in South China,accounting for 25.6,24.0 and 9.1%of total forested area in Guangdong Province,respectively(Fig.1)(State Forestry Administration of the People’s Republic of China 2014).Soils in this region are mainly laterite,lateritic red soil,and red soil.

Soil sampling and measurement

Fig.1 Sampling sites in Guangdong Province

Twenty-one sites,(in each site,PMPs and NBFs are adjacent to EPs),throughout Guangdong Province were chosen(Fig.1).Within each site,the soil type was exactly the same,altitude was nearly the same,and the age of EPs and PMPs was similar,having been established on former NBFs land.Both EPs and PMPs were in the first rotation(10–12 years-old).Ateach site,three random plots(10 m×10 m)were selected for the collection of soil samples,and three randomly located soil profiles were dug and samples of the A and B horizons were collected separately.Soils of the same horizon in the same plot were mixed together evenly,and a sample was collected from the mixture.Roots and stones in samples were removed using a 2 mm sieve.The samples were air-dried and stored for analysis.Three core samples(100 cm3volume)were collected from each horizon in every soil profile to determine bulk density and porosity(Carter and Gregorich 2008).

Bulk density,water-holding capacity,and porosity were determined by the core method,and hygroscopic moisture measured using the oven-drying method(Carter and Gregorich 2008).Soil pH was measured in deionized water(1:2.5 W/V)using a glass-calomel combination electrode.Cation exchange capacity(CEC)and exchangeable soil acidity were estimated by the compulsive exchange method and the potassium chloride method,respectively,and base saturation(BS)was calculated(Vogt et al.2015).Soil organic carbon(SOC)was determined by wet oxidation.Both total nitrogen(TN)and available nitrogen(AN)were measured by the modified Kjeldahl method.Extraction of total and available phosphorus was accomplished by digestion with H2SO4–HClO4and extraction with HCl–H2SO4,respectively,and measured using molybdenumblue.Total and available potassium were determined by digestion with HF and extraction with NH4OAc,respectively,and were quantified via atomic absorption spectrometry(AAS,HITACHI JACO6-25,Hitachi Ltd.,Japan).Available boron(B)was determined by the Azomethine-H method.

Statistics

All data were analyzed using Statistical Analysis Software(SAS 8.01,SAS Institute Ltd.,USA).One-way analysis of variance(ANOVA)was conducted to detect whether significant differences in soil properties existed among forest types.If these were detected(P＜0.05),Duncan’s multiple range test(DMRT)procedure was used to separate the mean values(α=0.05).

Results

General characteristics of EPs,PMPs and NBFs

Some general information including mean diameter at breast height,mean height,canopy density,litter depth,and the depth of A and B horizons were collected(Table 1).The data show that both EPs and PMPs had a shallower litter layer and a lower canopy density than NBFs.Conversely,EPs and PMPs had thicker A and B horizons than NBFs.

Soil physical and hydrological properties

Soil physical characteristics of bulk density,porosity,and moisture content were not significantly different between EPs and PMPs(Tables 2 and 3).Bulk density in the A horizon under EPs was significantly greater than under NBFs.However,total porosity,non-capillary porosity,total volume water,and hygroscopic water exhibited opposite results compared to that of bulk density,with values for these parameters being significantly lower in the A horizon beneath EPs compared to NBFs.EPs also showed significantly lower non-capillary porosity and hygroscopic water in the B horizon.Moreover,the ratio of non-capillary to capillary porosity was minimal in EPs,while it reached a maximum in NBFs.

Soil chemical properties

There were no significant differences in total and available phosphorous(TP,AP)and total potassium(TK)among EPs,PMPs,and NBFs(Tables 4 and 5).Differences between EPs and NBFs in the A horizon were significant for total and available nitrogen(TN,AN),available potassium(AK),and soil organic carbon(SOC).Compared with EPs,NBFs had 61.2,76.3,62.3,and 45.3%more TN,AN,AK,and SOC,respectively.In the B horizon,NBFs also had significantly higher levels of SOC,AN,and AK than EPs.The quantities of TN and SOC in both the A horizon and B horizon of PMPs were significantly lower than those of NBFs.Compared with PMPs,EPs had significantly higher boron levels in the A horizon and higher TN in the B horizon,but significantly lower AK contents in horizons A and B.

Soil pH,cation exchange capacity(CEC),and base saturation(BS)were not significantly different among EPs,PMPs,and NBFs(Table 5).What is noteworthy is that soils of EPs,PMPs,and NBFs were extremely acidic,with pH readings of 4.4–4.5.Moreover,CEC was＜10 cmol(+)/kg,and BS was＜50.0%in all EPs,PMPs,and BNFs,indicating that these soils had a poor capacity to buffer acidification.

Discussion

In our study,EPs and PMPs soils exhibited no significant differences in porosity or water retention capacities,as assessed by the evaluation of capillary and non-capillary porosities, capillary water, and hygroscopic water(Tables 2 and 3).However,compared with NBFs,both EPs and PMPs had lower non-capillary porosity and a lower ratio to capillary porosity (Table 2),indicating thatmoisture-holding capacity decreased in both EPs and PMPs relative to NBFs.This was further confirmed by EPs and PMPs having higher bulk densities and lower total volume water and hygroscopic water than NBFs(Tables 2 and 3).Liao et al.(2012)examined the differences in soil variables between plantations and adjacent natural forests from 73 published studies and found that soil moisture under a plantation cover decreased by 24.5%,while soil bulk density increased by 12.5%relative to natural forests.Further,results in our study showed that,compared with NBFs,both PMPs and EPs had lower canopy densities and litter depths(Table 1),and hence better moisture retention capacity because of greater canopy densities and litter depths,the first which can reduce soil damage by rain,and the second,soil evaporation by solar radiation(Liao et al.2012;Yang et al.2017).

Table 1 General characteristics of EPs,PMPs,and NBFs

Table 2 Soil bulk density and porosity of EPs,PMPs,and NBFs

Table 3 Soil water-holding capacity of EPs,PMPs,and NBFs

Table 4 Levels of TN,TP,TK,and SOC in soils of EPs,PMPs,and NBFs

The changes in soil nutrients and SOC have become major ecological aspects of EPs and PMPs that havereceived much attention(Bargali et al.1993;Li 2007;Wang et al.2013;Zhang et al.2015;Cook et al.2016).According to Wang and Wang(2017),change of soil water could affect formation of SOC and impact soil nutrient availability in EPs.We found that both EPs and PMPs had significantly lower levels of TN,AN,AK and SOC than NBFs,revealing that lower soil fertility occurred under EPs and PMPs(Tables 4 and 5).Studies from Chen et al.(2013)and Zhang et al.(2015)found that conversion from PMPs to EPs could decrease the availability of soil resources(SOC,TN,and AN)and alter the structure and function of the microbial community.Zhang et al.(2015)reported out thatEucalyptuscould absorb and convert large quantities of nutrients into biomass due to fast growth rates and a strong nutrient absorption capacity,which was presumably a primary reason for the depletion of nutrients and SOC observed under EPs.However,EPs and PMPs in our study did not differ significantly in N,P or K levels(Tables 4 and 5).Moreover,SOC as well as available boron which is basically released from litter decomposition,are higher in EPs than in PMPs in the A horizon(Guo et al.2015).SOC is a major driver of bulk density,porosity,nutrient contents,and cation exchange capacity(Nambiar and Harwood 2014).Several studies have shown that conversion of natural forests to plantations reduces the C input from the above-and belowground litter(Guillaume et al.2015;Cook et al.2016).

Table 5 Levels of AN,AP,AK,and B in soils of EPs,PMPs,and NBFs

We suggest that EPs and PMPs changed soil properties mainly because of the direct negative impacts on SOC.According to previous studies,EPs and PMPs suffered significant SOC losses due to a variety of reasons:(1)burning and/or removal of harvest residue and litter a common practice in EPs and PMPs in Guangdong Province,can seriously disturb soil microbial community composition and nutrient cycling(Chen et al.2013;Zhang et al.2015);(2)damage of understory vegetation when planting or harvestingEucalyptusorP.massonianaresults in biodiversity reduction and litter decomposition disturbance(Chen et al.2013;Zhang et al.2015;Kerr and Ruwanza 2016;Yang et al.2017);and,(3)plantation soils(mainly laterite,lateritic red soil,and red soil)with intensive weathering and leaching have low capacity to hold nutrients under a high temperate rainy climate,and this becomes more serious with the removal of forest litter and/or the damaging of under story vegetation(Qiu et al.2015).

There were no significant differences in pH,CEC,or BS among EPs,PMPs,and NBFs(Table 6).However,soils of these forests,particularly in NBFs,were extremely acidic with a poor capacity to buffer acidification.Acidic deposition,mainly as acid rain,is one of the most seriousecological problems in Guangdong Province(Department of Environmental Protection of Guangdong Province 2014).Long-term acidic deposition,especially N deposition,greatly contributes to significant soil acidification occurring in south China(Huang et al.2015).On the other hand,soils(mainly laterite,lateritic red and red soil)are highly weathered in this province often low in organic matter and buffering capacities(Tables 4 and 6)while high in leaching potential and aluminum availability.This weathering makes the soil more sensitive to acidic deposition(Qiu et al.2015).

Table 6 Soil pH,CEC and BS in EPs,PMPs,and NBFs

Conclusions

This study investigated the changes in soil properties,including hydrological characteristics,nutrient contents,pH,and soil organic carbon underEucalyptusplantations,Pinus massonianaplantations,and natural broad-leaved forests at a provincial scale.There were no significant differences in soil properties betweenEucalyptusand pine plantations but compared to natural forests,soils in both plantations had lower moisture retention capacities and nutrient contents.Moreover,soil acidification generally occurred in all cover types.We suggest thatEucalyptusas well asPinus massonianaaltered soil quality,possibly because there were negative impacts on soil organic matter.It is imperative to establish urgent measures aimed at improving soil quality in types of plantations.

AcknowledgementsWe gratefully acknowledge the Science and Technology Department of Guangdong Province,China(Nos.2015B020207002 and 2014A020216032)and the financial support of the National Natural Science Foundation of China(No.31270675).