Effects of nitrogen-phosphorus imbalance on plant biomass production: a global perspective

Background and aimsUnparalleled inputs of anthropogenic nitrogen (N) and phosphorus (P) cause a strong N-P imbalance in terrestrial ecosystems. However, the effects of N-P imbalance on plant biomass production remains unclear.MethodsGiven that tissue N:P ratio may serve as an indicator of plant N or P limitations, we compiled a dataset reporting aboveground biomass (AGB) and tissue N:P ratio simultaneously from worldwide N addition experiments and explored the relationship between the responses of AGB and tissue N:P ratio to N enrichment.ResultsThe N-induced changes in AGB exhibited an asymptotic relationship (i.e., Michaelis-Menten function) with changes in tissue N:P ratio, indicating a progressive P limitation with increasing N. Our results further revealed that plant N and P status was related to the changes in soil inorganic N and P concentrations. Soil N increased while soil P remained unchanged with increasing N rate, thus resulting in an unbalanced soil N and P as N continues to increase.ConclusionsThis study is the first to report the influences of human-induced N-P imbalance on plant biomass production at the global scale. The biomass-N:P ratio relationship needs to be considered for reliable predictions of the future global carbon dynamics under global change

Nitrogen availability determines ecosystem productivity in response to climate warming

One of the major uncertainties for carbon-climate feedback predictions is an inadequate understanding of the mechanisms governing variations in ecosystem productivity response to warming. Temperature and water availability are regarded as the primary controls over the direction and magnitude of warming effects, but some unexplained results signal that our understanding is incomplete. Using two complementary meta-analyses, we present evidence that soil nitrogen (N) availability drives the warming effects on ecosystem productivity more strongly than thermal and hydrological factors over a broad geographical scale. First, by synthesizing temperature manipulation experiments, a meta-regression model analysis showed that the warming effect on productivity is mainly driven by its effect on soil N availability. Sites with a higher warming-induced increase in N availability were characterized by stronger productivity enhancement and vice versa, suggesting that N is a limiting factor across sites. Second, a synthesis of full-factorial warming x N addition experiments demonstrated that N addition significantly weakened the positive warming effect, because the additional N induced by warming may not further benefit plant growth when N limitation is relieved, providing experimental evidence that N regulates the warming effect. Furthermore, we demonstrated that warming effects on soil N availability were modulated by changes in dissolved organic N and soil microbes. Overall, our findings enrich a new mechanistic understanding of the varying magnitudes of observed productivity response to warming, and the N scaling of warming effects may help to constrain climate projections

Pre- and Post-silking Carbohydrate Concentrations in Maize Ear-leaves and Developing Ears in Response to Nitrogen Availability

Maize (Zea mays L.) grain yield is considered to be highly associated with carbohydrate dynamics in leaves and developing ears during the critical period bracketing silking. Carbohydrate changes are sensitive to variation in nitrogen (N) availability, yet a comprehensive analysis of the N effect on various carbohydrate concentrations around silking remains elusive. A 2-yr field study was conducted to investigate grain yield, N uptake, ear dry matter and carbohydrate concentrations in ear-leaves and whole ears (prior to silking) and kernels (after silking) of maize grown with 0, 150, and 300 kg N ha(-1). Greater N availability increased maize shoot dry matter and N content at silking and physiological maturity, as well as grain yield. While N had little effect on ear-leaf glucose concentration, sucrose concentration increased but starch concentration decreased with increasing N, regardless of sampling time. Prior to silking, glucose and fructose concentrations in the developing ear responded positively to increasing N availability, but sucrose and starch concentrations declined. In growing kernels shortly after silking, glucose and fructose concentrations in N fertilized treatments were significantly lower than those in the zero-N treatment. In contrast, a significant increase in kernel starch concentration was found in response to 300 kg N ha(-1). These observations point to an important role of the carbohydrate composition of unpollinated ears prior to silking with regard to kernel set and post-silking kernel starch accumulation, and thus final crop yield