Effect Of Combined Amendment Of Wheat And Huai Bean Straws On Soil Organic Carbon Sequestration

Soil organic carbon is an important marker for soil fertility,and soil organic carbon sequestration plays an important role in agricultural sustainable development.Weibei Highland area is an important agricultural region in the Loess Plateau of China,but there are some problems with soil fertility in cropland in recent years.It is found that there is a special fertilization model to improve soil fertility in this area,which is keeping stubble after the harvest of winter wheat,and then planting leguminous green manure in the summer fallow period.But because of high temperature and drought in this period,the stubble will not decay immediately,but decomposes in the form of wheat and green manure mixed-straws after the stubble and green manure ploughing together in the winter wheat pre-sowing period.Existing research has found that intercropping with legumes is more conducive to soil organic carbon accumulation than cereal monoculture system,but the organic carbon sequestration mechanism under the system with the coexistance of cereal and legumes residues is still unknown.At the same time,it has been widely reported that when the residues are mixture,there would be a“mixing effects”appeare during the decomposition,and therefore has a negative or positive effects on plant residues decomposition and soil organic carbon mineralization.Accordingly,although there are a large number of studies on the relationship between the crop residues returning and soil organic carbon sequestration,soil organic carbon sequestration mechanisms of combined application of cereal and legumes residues is still unclear,and most of studies do not separate the organic carbon which is newly formed from the plant residues from soil native carbon.So raising the carbon sequestration potential of the combined application of cereal and legumes straws has an important significance on global carbon cycle research and agricultural sustainable development.In order to investigate the effects of combined addition of wheat and legume straws on soil organic carbon sequestration,the study based on the wheat and huai bean rotation systems of Changwu Shaanxi and combined field and incubation experiment,in which the field experiment with 6 treatments were set up,the treatments were:without fertilizer and straw?F,G?,addition of fertilizer but no straw returning?F+NPK,G+NPK?and both fertilizer and straw returning?F+S+NPK,G+S+NPK?under the conditions of summer fallow and leguminous green manure rotation;At the same time a 80-day incubation experiment with 8treatments which were control?CK?,wheat straw addition?W?,huai bean straw addition?L?,biochar addition?BC?,wood ash addition?A?,combined addition of wheat and huai bean straws?W+L?,combined addition of biochar and these two kinds of straws?W+L+BC?,combined addition of wood ash and these two kinds of straws?W+L+A?were conducted,in which the?¹³C technique combined with the double exponential model was applied.In this study,the effects of green manure rotation on yield of winter wheat,nitrate and ammonium nitrogen in 0-200 cm and DOC content in 0-40 cm soil layer were measured and then the changes of soil labile carbon and nitrogen under wheat?Triticum aestivum L.?and huai bean?Glycine ussuriensis Regel et Maack?rotation systems were investigated.At the same time,the soil organic carbon sequestration mechanism of the combined addition of straw was preliminarily revealed,and the ways to enhance carbon sequestration potential of combined addition of straw were further explored by studying the effects of combined addition of two different straws and in concert with biochar and wood ash on soil organic carbon mineralization,distribution of newly formed carbon in soil organic carbon pool,changes of the soil recalcitrant organic carbon pool and soil active carbon and nitrogen.The main results are as follows:1.The effects of short-term planting of leguminous green manure and wheat straw returning on wheat yield and soil labile carbon and nitrogen were studied through a year of fixed field experiment.The results showed that the wheat yield of green manure decreased compared with the summer fallow treatments.The water content of 0-200 cm in the green manure areas were lower than that in the summer fallow.Green manure planting increased the ammonium nitrogen content of the 0-80 cm soil layer.And on the basis of leguminous green manure planting and wheat straw returning,the DOC content of 0-10 cm could be increased significantly by 16.5%compared with bare land,which was beneficial to the improvement of the soil fertility.2.Study of incorporation both wheat and leguminous straws on soil organic carbon sequestration in dark loessial soils showed that the cumulative CO₂ emissions of the soil and the SOC were significantly increased by W+L by 5.63 folds and 21.7%,respectively,relative to the CK.The W+L treatment markedly increased the microbial biomass carbon?MBC?content and microbial entropy(q_MB)by 39.5 and 20.0%,respectively,compared to CK.In addition,compared with W,the W+L treatment could significantly increase the cumulative CO₂ emissions of the soil,C₂,C₂/SOC,MBC content,and q_MB,the increments being 84.4,28.1,23.9,25.3 and 21.1%,respectively.At the same time,W+L markedly increased the exogenous newly-formed carbon from crop straws(C_straw)and its proportion to the total organic carbon?f?,and the increases were 86.5%and 79.8%,respectively.Additive effects were observed in the soil accumulation mineralization,SOC,and MBN.3.Studies on wheat and leguminous straws combined with biochar for soil carbon sequestration indicated that although biochar combined with straw had little effect on soil mineralization rate and cumulative mineralization,soil microbial biomass carbon content could be decreased significantly by 27.5%,compared with the treatment of no biochar.Soil DOC content were significantly decreased by 13.1%,relative to straw added alone.In addition,the results of the kinetic simulation of carbon mineralization showed that the combined application of biochar promoted the content of soil potential mineralization carbon pool,soil organic carbon content and the soil recalcitrant organic carbon pool by 1.15,32.2and 1.70%respectively.4.The result of the incorporation of both wheat and leguminous straws and wood ash on soil organic carbon sequestration and soil labile carbon and nitrogen content showed that exogenous addition of wood ash inhibited the soil organic carbon mineralization in a certain extent,and reduced the soil cumulative mineralization compared with straw added individually.The combined application of wood ash significantly reduced soil microbial carbon and DOC content by 8.60 and 11.1%respectively.Furthermore,although the combined application of wood ash had little effect on soil organic carbon content and recalcitrant organic carbon pool,the retention time of newly formed carbon from straw was significantly increased by 92%.In summary,the main conclusions are as follows:?1?Green manure planting combined with wheat straw returning could significantly improve the content of soluble organic carbon of the topsoil,but fertilizing effect are small in the short term;?2?Soil organic carbon sequestration capacity of the mixed addition of cereal and leguminous straws is superior to cereal alone,and therefore has more advantages on the improvement of soil fertility;?3?Combined application of biochar and cereal and leguminous straw mixture could improve the stability of soil organic carbon pool.But when we replace biochar with wood ash,the turnover of exogenous newly formed organic carbon will be prolonged.Both biochar and wood ash are good for the improvement of soil carbon sequestration potential under the condition of the combined application of cereal and leguminous straws.