Search

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 β-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.

Understanding of feedbacks between plant N uptake and soil N transformations in different vegetable production systems is crucial to increase vegetable N acquisition and improve N fertilizer management. 15N tracing pot experiments were conducted under greenhouse (GH) and open-field (OF) conditions; plant N uptake and soil gross N transformation rates were simultaneously quantified using the Ntraceplanttool. The feedbacks of plant N uptake on rhizosphere soil gross N transformations differed between GH and OF. Higher plant NH4+and NO3–uptake rates were observed in GH compared to OF, which were associated with higher plant biomass in GH. Rhizosphere soil mineral N production rates (i.e., the sum of mineralization and heterotrophic nitrification rates) in GH ranged from 0.18 to 1.53 mg N kg−1day−1and were significantly lower than in OF (0.60 to 4.56 mg N kg−1day−1). The reduced rhizosphere soil mineral N production rates in GH were attributed to high soil moisture conditions and vegetable activities (especially respiratory activity in rhizosphere). Rhizosphere soil microbial N immobilization rates in GH were 1.6–166.5 times lower compared to OF, which was line with decreased mineral N production rates and enhanced plant N uptake rates. This was also confirmed by the fact that the microbial N immobilization rates showed a negative correlation with the mineral N production rates and the N uptake rates of plants. We showed that the capacity of soil rhizospheric mineral N supply was weaker in GH than OF, which was one of the reasons why more N fertilizer was required to achieve rapid vegetable growth in greenhouse vegetable production. We also highlighted the importance of water management to improve rhizosphere soil N supply in greenhouse vegetable cultivation.

Maize (Zea maysL.) N uptake affects N use efficiency (NUE), which is affected by the N form for uptake. However, maize N preference for NH4+or NO3−is unclear. A field experiment and a 15N tracing study was conducted with various soils differing in pH plant with maize. Maize N (NO3−and NH4+) uptake rate significantly varied with soil pH, but NO3−uptake rate (UNO3) was much higher than NH4+uptake rate (UNH4) irrespectively of the pH, i.e., maize generally preferred NO3−uptake over NH4+at the seedling stage. The generally higher UNH4and UNO3in acidic compared to alkaline soils in the laboratory study were consistent with the maize yield in the field experiment results. The positive relationship between UNO3and UNH4(P< 0.01) indicated a synergistic effect between NH4+and NO3−uptake. The longer NH4+residence time could explain the higher NH4+and total N uptake rates in acidic soil than alkaline soil. With maize planted, the rate of autotrophic nitrification (ONH4) reduced between 30 and 75% compared to unplanted soils supporting N uptake via a prolonged NH4+residence time. The rate of heterotrophic nitrification (ONrec) was stimulated by maize plants, accounting for up to 45% of total NO3−production in acidic soils. The role of ONrecwas illustrated by a positive correlation of ONrec/(ONH4+ ONrec) with UNO3(P< 0.01). Maize seedling preferred NO3−over NH4+irrespectively of pH. Moreover, maize can regulate soil N supply through interactions between plant and soil N transformations to meet their N acquisition.