Land Use Change from Natural Tropical Forests to Managed Ecosystems Reduces Gross Nitrogen Production Rates and Increases the Soil Microbial Nitrogen Limitation.

Understanding the underlying mechanisms of soil microbial nitrogen (N) utilization under land use change is critical to evaluating soil N availability or limitation and its environmental consequences. A combination of soil gross N production and ecoenzymatic stoichiometry provides a promising avenue for nutrient limitation assessment in soil microbial metabolism. Gross N production via 15N tracing and ecoenzymatic stoichiometry through the vector and threshold element ratio (Vector-TER) model were quantified to evaluate the soil microbial N limitation in response to land use changes. We used tropical soil samples from a natural forest ecosystem and three managed ecosystems (paddy, rubber, and eucalyptus sites). Soil extracellular enzyme activities were significantly lower in managed ecosystems than in a natural forest. The Vector-TER model results indicated microbial carbon (C) and N limitations in the natural forest soil, and land use change from the natural forest to managed ecosystems increased the soil microbial N limitation. The soil microbial N limitation was positively related to gross N mineralization (GNM) and nitrification (GN) rates. The decrease in microbial biomass C and N as well as hydrolyzable ammonium N in managed ecosystems led to the decrease in N-acquiring enzymes, inhibiting GNM and GN rates and ultimately increasing the microbial N limitation. Soil GNM was also positively correlated with leucine aminopeptidase and ß-N-acetylglucosaminidase. The results highlight that converting tropical natural forests to managed ecosystems can increase the soil microbial N limitation through reducing the soil microbial biomass and gross N production.

Carbon and nitrogen fractions control soil N2O emissions and related functional genes under land-use change in the tropics.

Converting natural forests to managed ecosystems generally increases soil nitrous oxide (N2O) emission. However, the pattern and underlying mechanisms of N2O emissions after converting tropical forests to managed plantations remain elusive. Hence, a laboratory incubation study was investigated to determine soil N2O emissions of four land uses including forest, eucalyptus, rubber, and paddy field plantations in a tropical region of China. The effect of soil carbon (C) and nitrogen (N) fractions on soil N2O emissions and related functional genes was also estimated. We found that the conversion of natural forests to managed forests significantly decreased soil N2O emissions, but the conversion to paddy field had no effect. Soil N2O emissions were controlled by both nitrifying and denitrifying genes in tropical natural forest, but only by nitrifying genes in managed forests and by denitrifying genes in paddy field. Soil total N, extractable nitrate, particulate organic C (POC), and hydrolyzable ammonium N showed positive relationship with soil N2O emission. The easily oxidizable organic C (EOC), POC, and light fraction organic C (LFOC) had positive linear correlation with the abundance of AOA-amoA, AOB-amoA, nirK, and nirS genes. The ratios of dissolved organic C, EOC, POC, and LFOC to total N rather than soil C/N ratio control soil N2O emissions with a quadratic function relationship, and the local maximum values were 0.16, 0.22, 1.5, and 0.55, respectively. Our results provided a new evidence of the role of soil C and N fractions and their ratios in controlling soil N2O emissions and nitrifying and denitrifying genes in tropical soils.