Effects of Crops,Soil Types and Irrigation Methods on Digestion Capacity of Biogas Slurry

Lihong GAO,Ping LI,Xiuli WEI,Yi ZHAI

Institute of Agricultural Engineering,Chongqing Academy of Agricultural Sciences,Chongqing 401329,China

Responsible editor:Xiaoxue WANG Responsible proofreader:Xiaoyan WU

Biogas slurry refers to liquids produced from anaerobic fermentation of animals’ manures and direct discharge clearly poses threats to eco-environment.At present,the harmless treatment and utilization of biogas slurry has become a key factor preventing sustainable development of biogas projects in China.From the perspectives of resource optimization and circular economy,it is the most direct and effective way to return biogas slurry into farmlands as a kind of organic fertilizers[1-3].Most researches available indicate that the application of biogas improves both yield and quality of crops[4-7].However,biogas slurry contains high chemical oxygen demand (COD),and rich organic matter,highly-concentrated nitrogen and phosphorus,so that the application of biogas slurry at a high quantity might result in excessive accumulation and even loss of nutrients,as well as secondary pollution[7-9].

The research took different soils and crops as objects and investigated the digestion capacity of biogas slurry in order to provide scientific references for rational use of biogas slurry.

Materials and Methods

Test materials

The test crops included Yuqingyu No.3 (silage maize),sweet sorghum and early-mature No.5 Chinese cabbage.The test soils were purple and yellow soils(Table1).

Table1 The nutrient contents of biogas slurry and different soils

Test methods

The research included pot-culture test and field test.Specifically,for potculture test,the test materials included silage maize,sweet sorghum and Chinese cabbage,and soils were yellow and purple soils.The pot was 32 cm in diameter and 35 cm in height.The required quantities of nitrogen for different crops were converted into the quantities of applied biogas slurry as follows:biogas slurry quantities of silage maize were set 6 gradients,including 0.16,0.26,0.36,0.46,0.56 and 0.76 pot/L; the quantity of sweet sorghum was set 0.86,0.96,1.06,1.16,1.26 and 1.36 pot/L; the quantity of Chinese cabbage was set 0.07,0.12,0.17,0.22,0.27 and 0.32 pot/L.In field,the test material was just Chinese cabbage and the soil was purple soil,as per flood irrigation and sprinkling irrigation.The area of test field was 1.0 m2.Subsequently,the required nitrogen quantity of Chinese cabbage was converted into biogas slurry to be applied,involving 5 gradients,as follows:0.86,1.50,21.3,2.75 and 3.39 L per test field respectively.The biogas slurry in the test was applied as base fertilizer,in seedling stage and growth stage respectively,and plant height,and fresh weight were measured in harvesting period.In control group,urea and phosphorus pentoxide containing the same quantity of P and N were applied.

Measurement methods

The contents of N,P and organic matter in soils and biogas slurry were measured as per Soil Agro-chemistrical Analysis[10].

Data processing

The test data were processed with Microsoft Excel 2003 and SPSS 10.0,and variance analysis and differences were performed with Oneway ANOVA and LSD.

Results and Analysis

Effects of treatments on silage maize and soil digestion capacity

Plant height is an important component at assessing crop appearance quality.As shown in Fig.1,as the application quantity of biogas slurry grew,the plant height of silage maize kept growing on either purple or yellow soils.The plant heights reached the peaks at 232.3 and 228.9 cm with biogas slurry (V1) at 0.76 L.However,after the treatment with biogas slurry at IV,the plant height slowed down in growth.Additionally,the plant height of silage maize on purple soils kept higher compared with yellow soils,possibly caused by high fertility of purple soils.

As shown in Fig.2,fresh weight of silage maize showed different trends with plant height with biogas slurry in the range of I to VI.On purple and yellow soils,fresh weight changed from increasing to decreasing.It is notable that on purple soils,fresh weight achieved the highest at 1.17 kg per plant with IV of 0.46 L.On yellow soils,however,the fresh weight achieved the highest at 1.13 kg per plant with V of 0.56 L.Based on the comparisons,it can be concluded that plant height reached the highest on purple soils when biogas slurry was VI,but fresh weight had reached the peak when biogas slurry was IV,showing significant differences (α=0.05) with other treatments.Similarly,plant height of silage maize on yellow soils achieved the peak with biogas slurry of VI,but fresh weight reached the highest with biogas slurry of V,when fresh weight showed significant differences with rest treatments (α=0.05).Considering yield of silage crops is generally weighted by fresh weight per unit area,the quantity of biogas slurry was taken as a standard when fresh weight of silage maize reached the peak.It can be computed that digestion rate of biogas slurry by purple soils reached 57 t/hm2and 70.5 t/hm2by yellow soils.

Effects of treatments on sweet sorghum and soil digestion capacity

As shown in Fig.3,as the application of biogas slurry increased,plant heights of sweet sorghum in two soils both kept growing.The plant height in purple and yellow soils both reached peaks at 165.7 and 160.8 cm with biogas slurry of 1.36 L.In addition,similar to silage maize,plant height of sweet sorghum maintained higher based on purple soils compared with yellow soils.

As shown in Fig.4,fresh weights of sweet sorghum kept growing in two soil types.When biogas slurry was 1.36 L,fresh weight of sweet sorghum reached the highest of 0.76 kg per plant in purple soils and 0.71 kg per plant in yellow soils.Generally speaking,fresh weight of sweet sorghum changed the most remarkably with biogas slurry quantities from Ⅱto V,followed by insignificant changes thereafter,which coincided with the changes of plant height of sweet sorghum.In different treatments,plant height,based on purple or yellow soils,both kept growing.However,on basis of multiple comparisons of fresh weight of sweet sorghum,it showed that the increase of plant height showed non-positive correlation with fresh weight in two soils when the quantity of biogas slurry reached higher.In purple soils,when the quantity of biogas slurry was at 1.36 L,the maximal fresh weight of sweet sorghum was 0.76 kg per plant,showing significant differences with rest quantities,except of V=1.26 L(α=0.05).In yellow soils,the change of sweet sorghum showed much similar.Therefore,the quantities of digested biogas slurry by purple and yellow soils were both 157.5 t/hm2when sweet sorghum was grown.

Effects of treatments on Chinese cabbage growth and soil digestion capacity

As shown in Fig.5,the changes of fresh weight differed in purple and yellow soils for Chinese cabbages from the quantities of I to VI.In purple soils,fresh weight of Chinese cabbage kept growing upon biogas slurry.When the quantity reached V of 0.27 L,the fresh weight achieved the highest of 78.0 g per plant,followed by decreasing.In yellow soils,the fresh weight of Chinese cabbage was always growing upon biogas slurry,and achieved the maximum of 79.5 g per plant with the quantity of VI of 0.32 L.besides,fresh weight changed significantly in treatments with biogas slurry in almost whole process except of the period fromⅤto Ⅵ.

In purple soils,when biogas slurry was atⅤof 0.27 L,the fresh weight of Chinese cabbage reached the highest of 78.0 g per plant.According to multiple comparisons,the fresh weights showed little differences with biogas slurry quantities at 0.27 and 0.32 L,but showed significant differences with rest treatments (α=0.05).In yellow soils,in contrast,the fresh weight of Chinese cabbage reached the maximum when biogas slurry was at Ⅵof 0.32 L,showing significant differences with rest treatments (α=0.05).Therefore,based on crop yield and rational fertilization,if Chinese cabbage was grown,the quantities of digested biogas slurry in purple and yellow soils were 34.5 and 40.5 respectively.

Effects of irrigation methods on Chinese cabbage growth and soil digestion capacity

As shown in Fig.6,by flood irrigation and sprinkling irrigation,fresh weights of Chinese cabbage on purple soils were growing upon biogas slurry and reached the highest at 2.11 and 2.04 kg per test region,with biogas slurry of 3.38 L.Specifically,with biogas slurry quantities of Ⅰto Ⅲ,fresh weight of Chinese cabbage showed higher in the treatment by flood irrigation,compared with sprinkling irrigation,but the fresh weight increased remarkably from biogas slurry quantities of Ⅲto Ⅳ,possibly caused by irrigation methods.

It can be concluded from multiple comparisons that by flood irrigation,fresh weight was the highest when biogas slurry was at V of 3.38 L,and significant differences existed among the treatments (α=0.05).By sprinkling irrigation,however,with biogas slurry at Ⅳof 2.75 L,fresh weight of 1.96 kg per test region showed none significant difference with the treatment with fresh weight of 2.04 kg per test region(α=0.05).Based on calculations,it can be concluded that the digestion of biogas slurry by purple soils can be 36 t/hm2by flood irrigation,and 27 t/hm2by sprinkling irrigation if Chinese cabbages are grown.

Discussions and Conclusions

The research explored the digestion capacities of different crops and soils on biogas slurry and the results showed that for the same soil type,the digestion capacity on biogas slurry tended to be volatile upon crop type(sweet sorghum ＞silage maize ＞Chinese cabbage) and for the same crop variety,the digestion capacities also differed upon soil types (purple soil＜yellow soil).Furthermore,irrigation methods also affected the digestion capacity of soils (flood irrigation＞sprinkling irrigation).Additionally,the fresh weights performed higher than that of control in the treatments with standard quantity of biogas slurry,suggesting fertilizer efficiency of biogas slurry is better than that of chemical fertilizer.Biogas slurry is a liquid fertilizer applied on farmlands,with excellent effects.Nevertheless,considering the absorption and digestion capacities of crops and soils are limited,excessive application of biogas slurry would result in yield decrease and even ecoenvironment pollution.Hence,biogas should be applied as per soil type and crop variety in a rational way.The research investigated digestion capacities of biogas by silage maize,sweet sorghum and Chinese cabbage in purple and yellow soils as per two irrigation methods in order to provide theoretical references for land digestions caused by manure pollutions of breeding industry in Three Gorges Reservoir Region.

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