Synergistic Effect And Mechanisms Of Improving Soil Carbon And Nitrogen Sequestration With Organic Materials In Black Soil Region Of Northeast China

Straw return is an effective practice for improving soil carbon and nitrogen sequestration in large parts of China,but long-term continuous straw return did not significantly increase the organic carbon and nitrogen stocks of black soils in Northeast China.Today there is a new focus on the protection and utilization of black soil.One approach that is being promoted is to plough straw back into the soil,sometimes with a rotary ploughing machine(i.e.soil replacement),or manure application,buildup deep cultivated layer and increase the level of organic matter in the soil.But the influencing degree of soil replacement,straw return and straw return combination with manure on soil carbon and nitrogen sequestration is not clear.Our study is based on long-term continuous straw return experiment sites in Northeast China and 13C and 15N labeling technique.This study tried to quantify the effect oflong-tern straw return on soil organic carbon,and reveal the mechanism in improving the sequestration ability of organic carbon and nitrogen under straw amendment methods.The objective of this proposal is to elucidate the changes in soil native carbon and nitrogen,straw carbon and nitrogen allocation,and to reveal the mechanism of soil carbon and nitrogen turnover with different treatments.The main conclusions showed as follows:1.Long-term field experiments were conducted at three sites in Northeast China with varying latitudes and SOC densities.The results showed that the SOC stocks resulting from NPKS treatment were 4.0%and 5.7%higher than those from NPK treatment at two sites,but straw return did not significantly higher soil carbon sequestration rates than the NPK treatment.The equilibrium value of the CSR for the NPKS treatment equated to cultivation times of 17,11,and 8 years at the different sites.Straw return did not significantly increase the SOC stocks in regions with low SOC densities,but did enhance the C pool in regions with high SOC densities.These results show that there is strong regional variation in the effects of straw return on the SOC stocks in black soil in Northeast China.Additional cultivations and fertilization practices should be used when straw return is considered as an approach long-term improvement of the soil organic carbon pool.2.In a 360-day study an in situ 13C&15N-tracing technique was used to elucidate the effects of straw burial depth(15 and 35 cm),manure(presence or absence)and their interaction on C and N sequestration.Soil organic carbon(SOC)and total nitrogen(TN)content were significantly higher in treatments including manure plus straw(SMS)compared to the soil(S),soil plus straw(SS)and soil plus manure(SM)treatments in both of the soil horizons studied.After 360 days,straw N sequestration was unaffected by burial depth,but significantly more straw C was sequestrated at the 35 cm soil depth compared with the 15 cm soil depth:39.9%vs.35.8%.The straw-derived C amounted to 37.9%for the SS treatments and it was lower,at 2.2-10.0%and 5.7-13.7%for the SMS treatments at depths of 15 and 35 cm,respectively.The mean residence time of the C and N was longer by,respectively,16 and 27 days at the 15 cm soil depths and 31 and 37 days at the 35 cm depth in the SMS treatments compared to the SS treatments.Hence,the SMS treatments improved stability of carbon and nitrogen in deep soil layers.We concluded that returning straw to deep soil layers increased straw C sequestration in soils;combining the application of manure with returning straw to deep soil layers improves the stability of organic carbon and nitrogen in soil without compromising the potential for carbon and nitrogen sequestration.3.Soil carbon and nitrogen turnover and the proportion of straw-C and N distributed in the soil under straw buried depth(15 and 35 cm),soil replacement,straw return and straw return combination with manure were studied with the buried nylon bags method and 13C and 15N labeling technique.It was found that compared to the control(no soil replacement),the soil organic carbon and total nitrogen content of topsoil replacement treatment increased by 3.2%and 0.9%,straw-derived C and N increased by 3.6%,the proportion of remaining straw C and N decreased by 5.8%and 6.2%.The soil carbon and nitrogen sequestration were improved in the topsoil replacement treatment.However,contradictory results have been obtained in the subsoil replacement treatment.