| Soil erosion and extensive management are the key factors restricting farmland sustainability on the Loess Plateau and seriously hinder the high-quality development of regional agriculture.Reasonable mulching measures can effectively improve soil fertility,enhance farmland productivity and comprehensive quality,which is a suitable field management model in hilly region of Loess Plateau.It is of great scientific significance for indepth understanding and evaluation of agroecosystem services to explore the effects of longterm mulching measures on element synergism and microbial nutrient utilization and the regulatory mechanism.At the same time,it provides theoretical basis for agricultural sustainability in hilly region of Loess Plateau.However,there is a lack of systematic research on soil resource supply and microbial nutrient utilization mechanism under mulching measures in Loess Plateau.Based on this,this study focused on the agroecosystem in hilly region of Loess Plateau.Based on the field positioning experiment of conservation tillage started in 2008,the management modes of no-mulching(NM),straw mulching(SM),plastic mulching(PM)and ridge-film mulching(RM)were selected.Based on the theory of ecological stoichiometry,field positioning monitoring,laboratory experiments and microbiological analysis were used to explore the effects of long-term mulcher measures on soil ecological stoichiometric characteristics and their balance with microbial nutrient requirements.At the same time,it was clarified that microorganisms regulate nutrient utilization efficiency,nutrient metabolism,and community structure through changes in extracellular enzyme stoichiometry to respond the changes of nutrient supply and requirement relationships.Combined with crop growth traits and rhizosphere-bulk soil microbial nutrient characteristics,the nutrient mutual feeding of crop-soil-microbial system and its mechanism on microbial nutrient utilization and sequestration were discussed.The main research results are as follows:(1)Long-term mulching measures significantly improved topsoil(0-30 cm)nutrient content and microbial biomass in hilly region of Loess Plateau,effectively alleviating the imbalance between soil resource availability and microbial nutrient requirements to improve yield.Under the accumulative effect of mulching,the contents of organic carbon,total nitrogen and total phosphorus in topsoil were significantly increased,and the contents of microbial biomass carbon,nitrogen and phosphorus were increased by 50.18%-69.53%,33.62%-65.86%and 18.18%-61.04%,respectively.SM and RM significantly reduced the soil stoichiometric ratio of available nutrients.Meanwhile,the response of microbial biomass to mulching varied with different sampling stages.The strict homeostasis made the microbial stoichiometric ratio not affected by nutrient stoichiometry(except total nutrient C:N),leading to effective mitigation of stoichiometric imbalance under PM and RM,which was mainly reflected in C:Nand C:P-imbalance.The synergism of carbon and phosphorus in topsoil under SM and RM significantly increased the yield by 54.06% and 62.43%,respectively.The regulation of stoichiometric imbalance was different between total and dissolved nutrients.Soil physical properties,available nutrient content and microbial biomass synergically drove 91% variation of stoichiometric imbalance under long-term mulching.(2)Long-term mulching measures reduced microbial respiration metabolism and improved microbial carbon utilization efficiency by secreting carbon and nitrogen acquiring enzymes,which was in response to the alleviation of stoichiometric imbalance in topsoil.The extracellular enzymatic stoichiometry models indicated that agricultural soil microbial metabolism was limited by carbon source energy and nitrogen nutrients.The explanation rates of soil stoichiometric imbalance for microbial carbon and nitrogen limitation variations were38.89% and 24.47%,respectively.SM and RM had positive effects on soil carbon and nitrogen acquiring enzyme activity and the stoichiometric ratio in topsoil,which helped to alleviate soil stoichiometric imbalance,thus reducing microbial nutrient limitation.At the same time,the alleviation of stoichiometric imbalance can effectively reduce microbial respiration metabolism and promote microbial nutrient utilization.The adaptive response of carbon use efficiency to soil stoichiometric imbalance was related to dissolved nutrients.Compared with total nutrients,the stoichiometric imbalance based on available nutrients and microbial respiration strengthened the regulation of nutrient use efficiency,jointly explaining the variation of 87.10% in nutrient use efficiency.(3)The topsoil microorganisms adaptively responded to the changes in soil microbial nutrient utilization through the enhancement of aggregation of fungal co-occurrence network and deterministic assembly process.Long-term SM significantly increased Chao1 index and between-habitat diversity of soil microbial community,and beta diversity of fungal community was regulated by extracellular enzymatic stoichiometric ratio and nitrogen use efficiency.The different fungal species among treatments were Ustilaginomycetes and Rhizophlyctidomycetes,and the bacterial species were Bacteroidia and Bacilli,which were respectively reflected in the positive effects of PM and RM.Stoichiometric imbalance regulated microbial community species composition by driving nitrogen use efficiency and explained 21.15% and 25.30% variation in dominant species components of fungi and bacteria.Mulching measures promoted the modular distribution of sensitive microbes and drove the aggregation of fungal networks under PM and RM through stoichiometric imbalance and extracellular enzymatic stoichiometry variation.The topsoil microbial community assembly gradually changed from stochasticity to deterministic process in response to the alleviation of soil C:P imbalance and changes in microbial nutrient utilization,in which the fungal community was dominant.(4)The variation of crop-soil resource availability and microbial nutrient requirements jointly regulated and promoted the balance of microbial nutrient supply and requirement in rhizosphere-bulk soil.Under long-term PM and RM measures,the hydrothermal conditions of rhizosphere soil and crop biomass were significantly improved,and the contents of available nitrogen and available phosphorus in rhizosphere-bulk soil of SM and RM were significantly increased.The variation of element content under mulching resulted in the response of nutrient stoichiometry and microbial stoichiometry in bulk and rhizosphere respectively.At the same time,under the regulation of crop relative growth rate and nutrient uptake rate during the key stage,the rhizosphere soil stoichiometry varied by 50.32%-82.60%,which was higher than the variation of 36.95%-68.05% in bulk soil.The alleviation effect of mulching on rhizosphere stoichiometric imbalance was not obvious compared with bulk soil.In addition,croprhizosphere synergistic regulated the availability of bulk soil resources,which effectively alleviated the bulk soil stoichiometric imbalance under mulching,and produced nutrient mutual feeding among crop-soil-microorganisms.(5)The mitigation of soil stoichiometric imbalance drove the accumulation of microbial necromass carbon and nitrogen in rhizosphere-bulk soil by improving microbial carbon use efficiency.The alleviation of rhizosphere-bulk stoichiometric imbalance significantly increased the utilization of carbon energy,while decreased the utilization of nitrogen nutrients.Different from the bulk soil,the variation of microbial carbon and nitrogen use efficiency in rhizosphere was significantly affected by the negative and positive linear effects of respiratory metabolism.The content of amino sugars in rhizosphere-bulk soil was significantly increased by 5.75%-92.59%,and the accumulation of microbial necromass was effectively increased by4.08%-116.67% and the contribution to the soil nutrient pool,especially in SM and RM.The variation of nutrient use efficiency under the control of soil stoichiometry promoted the accumulation of carbon and nitrogen from microbial necromass in rhizosphere-bulk soil,and the rhizosphere nutrient sequestration mainly depended on the contribution of fungal necromass.In conclusion,this study found that long-term RM measure could effectively improve soil resource availability of representative farmland in the hilly region of Loess Plateau,alleviate stoichiometric imbalance,and improve farmland productivity.Microorganisms responded to stoichiometric imbalance variation through the adjustment of metabolism,nutrient utilization and community structure,thus promoting soil nutrient sequestration and improving the agroecosystem services. |
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