| Carbon(C)and nitrogen(N)in agricultural soils are important to support sustainable agriculture and ecological construction.Management activities affect the stability and sustainable development of C and N pools in dryland agricultural ecosystems.Agricultural management is one of the important technical approaches to achieve soil C sequestration and emission reduction in dryland ecosystems.As widely used management practices in agroecosystems,cropping system and fertilization have strong impacts on soil microorganisms and microbial-mediated processes due to changes in soil conditions and available substrates,and then affect soil carbon and nitrogen cycling.Focusing on productivity improvement and ecological construction needs on the Loess Plateau,we determined the long-term effect of cropping systems and fertilization on soil C and N pools in dryland ecosystems of the Loess Plateau.Our experiment included three cropping systems,that is,continuous wheat as a nonlegume system,continuous alfalfa as a legume system,and a pea-winter wheat-millet rotation as a grain-legume rotation system.We also included a fallow system as a control to compare the effects of the cropping systems.Each of the three cropping systems received various fertilization treatments(including chemical fertilizers and manure fertilizers).Herein,we measured organic C(OC),N,labile OC and microbial residues C concentrations in soil samples(0-20 cm)from 1985 to 2019 to evaluate C and N dynamics in top soils.We also examined changes of soil OC(SOC),inorganic C(SIC),N,labile OC,mineral N and microbial residues C concentrations,p H and moisture content along 0-300 cm depth after 35 years fertilization.Redundancy analyses were used to figure out the relationship between among soil C and N fractions.This study systematically investigated the effects of fertilization on soil C and N pools in top and deep soil and its response to the cropping systems.The study also revealed to microbialrelated C and N accumulation process under different cropping systems and fertilization treatments.This study provides a theoretical basis for accurately assessing C budget,improving productivity,reducing negative effects,and establishing a sustainable ecological environment.Main results are as follow:(1)Long-term continuous alfalfa cropping system and fertilization were beneficial to increase the SOC and N contents and its availability and stability in top soils,and the effects of fertilization varied with cropping systems.In continuous alfalfa cropping systems,phosphorus fertilizer(P)has the potential to increase SOC and N contents in top soils.Compared with continuous winter wheat and grain-legume roation system,the continuous alfalfa cropping system increased the contents of SOC,N,potassium permanganate oxidized C(POXC)and microbial residue C,management index(CMI),the ratio of fungal residue C to bacterial residue C and contribution of fungal residues C to SOC in top soils from 1985 to 2019.Therefore,the continuous alfalfa cropping system could promote the accumulation of SOC by increasing soil fungal residue C.The P and nitrogen(NP)and NP and manure(NPM)significantly increased the contents of SOC,N,POXC and microbial residue C,and CMI in top soils,and the effects of NPM were greater than those of NP.However,the effects of NP or NPM on these variables in top soils were similar among the cropping systems.The effects of P on the contents of SOC,N,POXC and microbial residue C in top soils from 1985 were greater in the continuous alfalfa system but smaller in the continuous winter wheat system in comparison with those in the grain-legume rotation system.The P fertilizer was recommended for the continuous alfalfa cropping systems.(2)Long-term application of manure fertilizer divergently affects SOC and SIC in deep soil and this effect varies with cropping system.Manure increased SOC and SIC in top 80 cm layer but decreased SIC in 80-300 cm layer among the cropping systems,which offset increase of SOC and SIC and resulted in 50.01 Mg hm-2 decrease of total C in 0-300 cm soil layer.The effects of manure on SIC loss were greater in the continuous alfalfa system in comparison with those in the grain-legume rotation and continuous winter wheat systems.Given the importance of soil C for sustainable agriculture and global C cycle and that drylands contain 80% of the global soil inorganic carbon,immediate attention should be paid to such divergent effects in both mechanisms understanding and model prediction.(3)Long-term application of fertilizers increased soil labile OC content in 0-300 cm layer,resulting in downward leaching of nitrate.The accumulation of nitrate in deep soil be related to soil dissolved organic C and N,SOC and N.The NP and NPM also increased nitrate content in 0-300 cm layer,resulting in downward leaching of nitrate.The effects of NPM on nitrate were greater than those of NP.The effect of NP or NPM on nitrate varied with cropping systems.The continuous alfalfa cropping system with deep root could reduce the downward leaching of nitrate.The NPM increased POXC,dissolved OC,dissolved organic N contents and CMI in 0-300 cm layer,and the increasing effect decreased with soil depth.The effect of NPM on dissoved organic C and N in 0-300 cm layer were similar among the cropping systems.There was a significant positive correlation between dissolved organic C and N,SOC and N and nitrate in 0-80 cm soil layer.The accumulation of nitrate in deep soil may be related to the increase of dissolved organic C and N,SOC and N in 0-80 cm soil layer.The effects of P on POXC contents and CMI in 0-300 cm layer were greater in the continuous alfalfa system in comparison with those in the grain-legume rotation and continuous winter wheat systems.(4)Long-term fertilization can improve the stability of SOC and this effect varies with cropping system.Long-term fertilization increased the microbial residue C content in 0-80 cm soil layer.The NPM and NP treatments increased the microbial residue C content in 0-80 cm soil layer,and the effects of NPM on microbial residue C in 0-80 cm layer were greater than those of NP.The NP and NPM increased the contribution of microbial and fungal residue C to SOC in 0-300 cm soil layer,but had no effect on the contribution of bacterial residue C to SOC in 0-300 cm soil layer.The NP resulted in 12.9% and26.2% increases in the contribution of microbial and fungal residue C to SOC in 0-300 cm soil layer,respectively,while the NPM resulted in 19.7% and 32.0% increases,respectively.The effect of NPM or NP on microbial residue C and its contribution to SOC in 0-300 cm layer were similar among the cropping systems.The effects of P on the contribution of microbial and fungal residue C to SOC in 0-300 cm layer were greater in the continuous alfalfa system and the grain-legume rotation system in comparison with those in the continuous winter wheat system.Therefore,application of P fertilizer in legume-inluded cropping systems could promote SOC accumulation by increasing fungal residue C in 0-300 cm soil layer.(5)Soil OC was the most important factor affecting the accumulation of SOC in top soil,while soil IC was the most important factor in deep soil.Soil N were affected by both soil labile and non-labile C.Soil chemical properties and fractions of C and N explained 93.12% and 93.04%of the variations in the changes of C and N pools in top and deep soil,respectively.Among all variables,SOC explained the largest variation in soil C pool in top soil(0-20 cm),and SIC explained the largest variation in soil C pool in deep soil(20-300 cm).Among all variables,POXC and fungal residue C explained the largest variation in the changes of N pools in top and deep soil,respectively.The effects of fertilization on soil C and N pools in top and deep soil and its response to the cropping systems were elucidate.Long-term continuous alfalfa cropping system was beneficial to increase the SOC and N contents and its availability and stability in top soils.The effects of fertilization on soil C and N and its fraction in top and deep soils varied with cropping systems.In the continuous alfalfa cropping systems,P fertilizer has the potential to increase the SOC and N contents and its availability and stability in top and deep soils.In this study,SOC was the most important factor affecting the accumulation of SOC in top soil,whereas SIC was the most important factor in deep soil.Soil N were affected by both soil labile and nonlabile C.This study deeply analyzed the impact mechanism of fertilization on soil C and N in top and deep soils under different cropping systems,which is helpful for a more comprehensive understanding of soil C and N cycle in dryland ecosystems.It also provides a scientific basis for productivity improvement and ecological environment construction in dryland ecosystems. |
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