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Many croplands in the tropics of China have been converted over the last decades into areca nut plantations due to their high economic returns. This land-use transition was accompanied by changes in agricultural practices such as soil moisture regimes and fertilizer inputs, which may affect soil organic carbon (SOC) and its fractions, especially in tropical soils with low fertility and high nitrogen loss. Yet, how the time since land-use transition from rice paddy cultivation to areca nut plantations affects soil carbon dynamics and their underlying mechanisms in the tropics of China remains elusive. Here, areca nut plantation soils with different ages (2, 5, 10, 14, and 17 years) and paddy fields in the tropical region of China were investigated. The study result indicates that the contents of dissolved organic carbon (DOC), particulate organic carbon (POC), easily oxidized organic carbon (EOC), light organic carbon (LFOC), and microbial biomass carbon (MBC) decreased significantly with increased time since land-use transition from rice paddy cultivation to areca nut plantations. Similarly, the ratios of DOC/SOC, MBC/SOC, POC/SOC, LFOC/SOC, and EOC/SOC decreased significantly with increased time since land-use transition. Compared with the paddy soil, the carbon pool management index decreased by 36.6–76.7% under the areca nut plantations, concluding that increasing the time since land-use transition from rice paddy cultivation to areca nut plantations with high application rates of chemical fertilizers resulted in reduced soil active carbon fractions and SOC supply capacity. Therefore, agricultural practices such as the use of organic fertilizers should be applied to improve the soil's ability to supply organic carbon in managed plantation ecosystems in the tropics of China. [ABSTRACT FROM AUTHOR]

It is not clear how the number of 15 N-injection needles affects the estimated results of gross N transformation rates in the presence of plants. To evaluate this, a 15 N tracing study of sandy loam soil and a silty loam soil in the presence of maize (Zea mays L., cv. "Zhengdan 958") was conducted and the Ntraceplant tool was used to quantify gross N transformation rates. Our results showed that the number of 15 N-injection needles could significantly influence the estimated results of gross N transformation rates in the studied soil–plant systems. There was no discernible difference in the gross rates of N mineralization (Min), the oxidation of NH4+ to NO3− (ONH4), the oxidation of recalcitrant organic-N to NO3− (ONrec), the NH4+ and NO3− plant uptake between 4-needle and 6-needle injections for both studied soils. However, for the 1-needle and 2-needle injections in the silty loam soil, Min was significantly lower than the 4-needle and 6-needle injections. While, in sandy loam soil, Min measured with 1-needle injection was significantly higher than the 4-needle and 6-needle injections. The ONH4 and ONrec also varied for the 1-needle and 2-needle injections, compared to the 4-needle and 6-needle injections. The total plant N uptake rate declined with the increase in the number of injection needles. Based on these results and considering the simplicity of the experimental procedure, we suggest that the 4-needle injection could be used in the 15 N tracing studies conducted in pot (diameter = 6.4 cm in this study) experiments with plants, i.e., one needle for about 8 cm2. [ABSTRACT FROM AUTHOR]

Identifying tipping points in the relationship between aridity and gross nitrogen (N) cycling rates could show critical vulnerabilities of terrestrial ecosystems to climate change. Yet, the global pattern of gross N cycling response to aridity across terrestrial ecosystems remains unknown. Here, we collected 14,144 observations from 451 15N‐labeled studies and used segmented regression to identify the global threshold responses of soil gross N cycling rates and soil process‐related variables to aridity index (AI), which decreases as aridity increases. We found on a global scale that increasing aridity reduced soil gross nitrate consumption but increased soil nitrification capacity, mainly due to reduced soil microbial biomass carbon (MBC) and N (MBN) and increased soil pH. Threshold response of gross N production and retention to aridity was observed across terrestrial ecosystems. In croplands, gross nitrification and extractable nitrate were inhibited with increasing aridity below the threshold AI ~0.8–0.9 due to inhibited ammonia‐oxidizing archaea and bacteria, while the opposite was favored above this threshold. In grasslands, gross N mineralization and immobilization decreased with increasing aridity below the threshold AI ~0.5 due to decreased MBN, but the opposite was true above this threshold. In forests, increased aridity stimulated nitrate immobilization below the threshold AI ~1.0 due to increased soil C/N ratio, but inhibited ammonium immobilization above the threshold AI ~1.3 due to decreased soil total N and increased MBC/MBN ratio. Soil dissimilatory nitrate reduction to ammonium decreased with increasing aridity globally and in forests when the threshold AI ~1.4 was passed. Overall, we suggest that any projected increase in aridity in response to climate change is likely to reduce plant N availability in arid regions while enhancing it in humid regions, affecting the provision of ecosystem services and functions. [ABSTRACT FROM AUTHOR]