Abstract
The enzymatic activities and ratios are critical indicators for organic matter decomposition and provide potentially positive feedback to carbon (C) loss under global warming. For agricultural soils under climate change, the effect of long-term warming on the activities of oxidases and hydrolases targeting C, nitrogen (N) and phosphorus (P) and their ratios is unclear, as well as whether and to what extend the response is modulated by long-term fertilization. A 9-year field experiment in the North China Plain, including an untreated control, warming, N fertilization, and combined (WN) treatment plots, compared the factorial effect of warming and fertilization. Long-term warming interacted with fertilization to stimulate the highest activities of C, N, and P hydrolases. Activities of C and P hydrolase increased from 8 to 69% by N fertilization, 9 to 53% by warming, and 28 to 130% by WN treatment compared to control, whereas the activities of oxidase increased from 4 to 16% in the WN soils. Both the warming and the WN treatments significantly increased the enzymatic C:N ratio from 0.06 to 0.16 and the vector length from 0.04 to 0.12 compared to the control soil, indicating higher energy and resource limitation for the soil microorganisms. Compared to WN, the warming induced similar ratio of oxidase to C hydrolase, showing a comparable ability of different microbial communities to utilize lignin substrates. The relationship analyses showed mineralization of organic N to mediate the decomposition of lignin and enzyme ratio in the long-term warming soil, while N and P hydrolases cooperatively benefited to induce more oxidase productions in the soil subject to both warming and N fertilization. We conclude that coupled resource limitations induced microbial acclimation to long-term warming in the agricultural soils experiencing high N fertilizer inputs.
Highlights
Global warming due to anthropogenic activities poses an ongoing concern for increased soil respiration and emission of carbon dioxide to the atmosphere[1]
The total nitrogen (TN) contents decreased 5 to 14%, on average, at all sampling times, in soils subject to warming compared to the CTRL and the effect was significant in May, resulting in a subsequent significant increase in the ratio soil decomposable organic carbon (SOC)/TN in August
Nitrate contents (NO3−) significantly increased in December and May due to the interacting effect of warming and N fertilization and increased by long-term warming in August, compared to CTRL, indicating accelerated N mineralization and nitrification, with concomitant decrease in soil water contents (SWC) for about 3–4%, whereas ammonium contents ( NH4+) and soil pH were affected to a lesser degree by the treatments (Fig. 1)
Summary
Global warming due to anthropogenic activities poses an ongoing concern for increased soil respiration and emission of carbon dioxide to the atmosphere[1]. Microbial respiration is the essential constitute of soil respiration[4] and the acclimation of the soil microorganisms to long-term warming is assumed to result from the depletion of soil water and labile carbon (C), which limits the microbial a ctivities[6,7] and shifts the community to utilize complex C compounds such as lignin[4,8,9]. Soil microbes meet their nutrient demands by producing a variety of functional enzymes to catalyze the mineralization of diverse organic matter. Following the initial assumption of preferential utilization of labile C under long-term warming[4], it was hypothesized that (1) long-term warming increase oxidase activities to meet microbial nutrient demands, reflecting microbial nutrient demands better than C hydrolase, and (2) N fertilization stimulate higher oxidase activities by alleviating soil N limitation, or decreased oxidase and C-, N- and P-acquiring hydrolase activities by exacerbating drought in the soil exposing to long-term warming
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