Effects Of Moisture And Temperature On Soil Carbon, Nitrogen And Phosphorus Transformation And Corresponding Mechanisms

Drying-rewetting and freezing-thawing are two of the most common forms of abiotic perturbations experienced by soils.In arid and semiarid regions or in intensive production system,surface soils often undergo gradual drying by evapotranspiration followed by rapid rewetting as a result of precipitation or irrigation.Soil freezing-thawing largely occur in northern China in late Autumn or early Spring,when the daytime temperature is above zero,while the nighttime temperature is far below zero.Drying-rewetting and freezing-thawing can largely affect soil physical-chemical and biological properties,and microbial activities and community composition,and consequently result in changes in nutrient transformation and transport.Previous studies about nutrient transformation driven by drying-rewetting and freezing-thawing have been well documented,however,the studies that focused on the effects of drying intensity and the combined effects of drying-rewetting,environmental temperature and phosphorus?P?fertilizer application on soil carbon?C?,nitrogen?N?,P transformation were rarely reported,and the effects of drying intensity and freezing-thawing on the availability of extractable organic P were not clear.In the present study,cropland,grassland and forest soils from northern China were employed to examine the effects of straw amendment,P addition and temperature on P availability,microbial community composition and aggregate size distribution during drying-rewetting,and effects of drying intensity and freezing-thawing on soil C,N,and P transformation,and the phosphatase hydrolyzable P species.The main results were listed as follows:?1?We analyzed the nutrient composition and microbial communities of soils exposed to various drying intensities[(4%?10%?20%?30%?40%?60%water holding capacity?WHC?].We also used a sterilization experiment to examine the contribution of soil microbes to nutrient pulses.Soil drying-rewetting decreased C mineralization by 927%.Both monosaccharide and mineral N contents increased at higher drying intensities??20%WHC?and increased by 204%and 110%in the soils dried to 4%WHC,respectively,whereas labile P only increased?by 105%?when the soil was dried to 4%WHC.Moreover,levels of microbial biomass C and N and dissolved organic N decreased with increasing drying intensity and were correlated with increases in dissolved organic C and mineral N,respectively,whereas the increases in labile P were not consistent with reductions in microbial biomass P.The sterilization experiment results indicated that microbes were primarily responsible for the C and N pulses,whereas non-microbial factors were the main contributors of the labile P pulses.Phospholipid fatty acid analysis indicated that soil microbes are highly resistant to drying-rewetting events and that drought-resistant groups were likely responsible for the nutrient transformation.Therefore,the present study demonstrates that moderate soil drying could improve the mineralization of N but not P and that different mechanisms are responsible for the C,N,and P pulses observed during drying-rewetting events.?2?Bicarbonate-extractable P fractions?NaHCO₃-P?were analyzed in five soils exposed to various drying intensities?5%,10%,20%,30%,40%,and 50%WHC?,as well as hydrolyzable Po fractions via enzymatic hydrolysis?alkaline phosphatase,phosphodiesterase,and phytase,alone or in combination?.Bicarbonate-extractable Po accounted for 33.3-56.4%of extractable total P,and hydrolyzable Po represented 34.4-79.7%of extractable Po.For extractable Po,7.7-29.9%was labile monoester P,6.5-14.9%was diester P,and 17.8-36.5%was phytate-like P.Bicarbonate-extractable inorganic P and Po contents were not affected by 20-40%WHC treatments,but increased by 26.3-48.1%and 5.7-52.9%as the soils were dried to 5%WHC,respectively.Similarly,labile monoester,diester,and phytate-like P contents increased by 12.5-89.8%,0-65.2%,and 24.6-65.6%in 5%WHC soils,respectively.Pearson's correlation analyses showed that proportional increases in bicarbonate-extractable inorganic P and phytate-like P following extreme drought were positively correlated with soil organic carbon,oxalate-extractable aluminum,and iron,probably indicating the regulatory roles of organic matter and Al/Fe oxides in labile P release during soil drying-rewetting.Overall,our results highlight that extreme drought events probably result in the release of labile P,including substantial amounts of hydrolyzable Po fractions,thus potentially threatening water bodies in the context of climate change.?3?Five soils with contrasting physical-chemical properties were used to examine the effects of freezing-thawing cycles on NaHCO₃-Pi and NaHCO₃-Po.and evaluate the hydrolyzable Po fractions via enzymatic hydrolysis.Freezing-thawing cycles resulted in increases in NaHCO₃-Pi and Po by 0?73.8%and 0?41.1%,respectively.Freezing-thawing cycles increaseed labile monoester P and phytate-like P by 0?62.4%and 0?63.7%,respectively.The release of labile P during freezing-thawing was largely controlled by soil organic matter,Fe/Al oxides contents and land use type.?4?The combined effects of P fertilizer and temperature on P transformation and microbial community composition in a vegetable soil during successive four drying-rewetting cycles were examined.Soil drying-rewetting increased NaHCO₃-Pi content in control and swine manure treatment,but had little effects on NaHCO₃-Pi content in KH₂PO₄ treatment.NaHCO₃-Po content was significantly increased during soil drying-rewetting across P fertilizer and temperature treatments,and a positive interaction effect was observed during the 2nd?4th cycle in swine manure amended soils.Swine manure application significantly increased soil alkaline phosphatase activities,compared with KH₂PO₄ and control treatment.Higher temperature increased soil alkaline phosphatase activities,which was much higher than KH₂PO₄ and control treatment after one drying-rewetting cycle.Contents of monounsaturated and polyunsaturated fatty acids were increased after swine manure application,while higher temperature resulted in an increase in relative abundance of fatty acids 18:1?7c,18:1?9c,and 18:2?6,9c.Therefore,P fertilizer and temperature resulted in contrasting effects on NaHCO₃-P content during soil drying-rewetting,that is,swine manure application increased labile P release through enhanced alkaline phosphatase activitie and the abundance of heterotrophic communities,such as actinomycetes and fungi,and those characterized by phosphate-solubilizing bacteria?Pseudomonas spp.are indicated by fatty acid 18:1?7c?.Higher temperature resulted in drying-rewetting-induced P release via enhanced alkaline phosphatase activitie and Pseudomonas spp.abundance.?5?To examine whether organic amendments could improve the resistance and resilience of microbial function?extracellular enzyme activities?,community composition?phospholipid fatty acids?,and soil structure to drying-rewetting alternation,cropland soils with or without wheat-straw amendment were allowed to desiccate in a microcosm for two months,followed by moist incubation for five weeks,and continuously moist treatments were maintained at 50%water holding capacity during the entire period,as a control treatment.Straw amendment increased microbial biomass,extracellular enzyme activities,the relative abundance of fungal groups,dissolved organic carbon,and proportion of large macroaggregates?>2000?m?,but decreased mineral nitrogen and available phosphorus.The drying-rewetting treatment increased microbial biomass C and?-glucosidase activities by 10%and 13%in straw-amended soils,respectively,but not in unamended soils,and decreased the urease and alkaline phosphomonoesterase activities by>15%in unamended soils,but not in amended soils.The contents of fungi,actinomycetes,Pseudomonas spp.,and Bacillus spp.decreased with drying,and more so with the subsequent rewetting,but recovered by the end of the experiment.The drying-rewetting treatment caused a decrease in the nitrate content in both soils?>10%?and an increase in the macroaggregates of straw-amended soils?-8%?.These results indicated that improved soil aggregation,as a result of straw amendment,protected microbial communities from drought stress and that nutrient acquisition promoted the post-rewetting colonization of heterotrophic communities characterized by hydrolase production,which consequently facilitated aggregate re-formation.Thus,straw amendment positively contributed to aggregate turnover and to both microbial and enzymatic responses to drying-rewetting events,which suggests that straw amendment is favorable to maintain soil function under conditions of increasing rainfall variability.