| Soil hydrolases are the proximal drivers of organic matter decomposition via mineralizing organic matter and releasing carbon and nutrients, which can subsequently assimilated and utilized by plants and soil organisms. This process would be directly or indirectly affected by the global change, and thus affect the energy flow and material cycles in terrestrial ecosystems.The main purpose of this study was to investigate the changes of soil hydrolytic enzyme activity through a completely randomized experimental design with N addition, increased precipitation and their combinations in a Stipa baicalensis meadow steppe in northeastern China. The experimental station was established in 2010. The nitrogen we employed was NH4NO3 with three levels(N0: 0g N m-2a-1, N1: 5g N m-2a-1, N2: 10 g N m-2a-1) and added at the beginning of June from 2010 to 2014. One or two iron boxes were applied to added a certain proportion of the natural precipitation with three levels(W0: 0, W1: 1/7, W2: 2/7). Three soil samples(0 to 15 cm depth) were randomly collected and combined at each plot in May, July, August and September 2014. A rapid and highly sensitive method for measuring hydrolytic enzyme activities using 4 – methylumbelliferone- linked substrates(4MUB) and 7 – amino – 4 – methylcoumarin- linked substrates(AMC) fluorogenic substrates in a microplate system, including α-glucosidase(αG), β-glucosidase(βG), β-xylosidase,(βX), Cellobiohydrolase(CBH), N-acetylglucosaminidase,(NAG), Leucine aminopeptidase(LAP) and acid phosphatases(a P). We also measured soil properties, plant biomass and microbial biomass carbon. We aimed to detect: 1) what effects of different levels of N addition and increased precipitation on soil hydrolase activity during plant growth peak? 2) What impacts of nitrogen addition and increased precipitation on seasonal variation trend of soil hydrolase activity? 3) Which are the main environmental factors that influence the soil hydrolase activity under nitrogen addition and increased precipitation? Major conclusions are as follows:(1) Low N concentration(5g N m-2a-1) significantly increased soil hydrolase activity during plant growth peak, but higher N concentration weakened this promotion. During plant growth peak, low levels of increased precipitation(1/7 natural precipitation) improved βG, LAP and a P activities, while high levels of increased precipitation(2/7 natural precipitation) inhibited αG, βG, NAG, LAP and a P activities.(2) The interaction of N addition and increased precipitation had significant effects on αG, βG, NAG, LAP and a P activities during plant growth peak. But the promotion of hydrolase activity caused by N addition offset by increased precipitation.(3) N addition increased soil hydrolase activity in different seasons but did not change the seasonal variation of most hydrolase activity. Increased precipitation inhibited soil hydrolase activity in different seasons and reduced the variation range of hydrolase activity in different seasons.(4) Soil hydrolase activity were more sensitive to the alters of abiotic factors(e.g. nitrogen and phosphorus) more than biotic factor(e.g. plant and microorganism).(5) There is a threshold of N addition effect on soil hydrolase activity. Moderating N addition can promote hydrolase activity, while it would be inhibited effect when exceed the threshold. The regulation of precipitation on soil hydrolase was heavily influenced by the original soil moisture status and total rainfall of the year.We found that moderate N addition could promote hydrolase activities, but increased precipitation can neutralize or suppress this impact. In addition, the response of soil hydrolase activities to global change is largely dependent on the original nutritional status and rainfall pattern in this region. Furthermore, the responses of hydrolases to N addition and increased precipitation may have important implications in circulation state of the corresponding nutrient elements, which would be a good indicator of the soil nutrient status in global change in the future. |
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