Nitrogen response efficiency increased monotonically with decreasing soil resource availability: a case study from a semiarid grassland in northern China

Abstract

The concept of nutrient use efficiency is central to understanding ecosystem functioning because it is the step in which plants can influence the return of nutrients to the soil pool and the quality of the litter. Theory suggests that nutrient efficiency increases unimodally with declining soil resources, but this has not been tested empirically for N and water in grassland ecosystems, where plant growth in these ecosystems is generally thought to be limited by soil N and moisture. In this paper, we tested the N uptake and the N use efficiency (NUE) of two Stipa species (S. grandis and S. krylovii) from 20 sites in the Inner Mongolia grassland by measuring the N content of net primary productivity (NPP). NUE is defined as the total net primary production per unit N absorbed. We further distinguished NUE from N response efficiency (NRE; production per unit N available). We found that NPP increased with soil N and water availability. Efficiency of whole-plant N use, uptake, and response increased monotonically with decreasing soil N and water, being higher on infertile (dry) habitats than on fertile (wet) habitats. We further considered NUE as the product of the N productivity (NP the rate of biomass increase per unit N in the plant) and the mean residence time (MRT; the ratio between the average N pool and the annual N uptake or loss). The NP and NUE of S. grandis growing usually in dry and N-poor habitats exceeded those of S. krylovii abundant in wet and N-rich habitats. NUE differed among sites, and was often affected by the evolutionary trade-off between NP and MRT, where plants and communities had adapted in a way to maximize either NP or MRT, but not both concurrently. Soil N availability and moisture influenced the community-level N uptake efficiency and ultimately the NRE, though the response to N was dependent on the plant community examined. These results show that soil N and water had exerted a great impact on the N efficiency in Stipa species. The intraspecific differences in N efficiency within both Stipa species along soil resource availability gradient may explain the differences in plant productivity on various soils, which will be conducive to our general understanding of the N cycling and vegetation dynamics in northern Chinese grasslands.

Global-scale latitudinal patterns of plant fine-root nitrogen and phosphorus

“…1) and leaf latitudinal N:P patterns 6,8 , appear to support the geochemical hypothesis 11,14,20 , that is, tropical soils are on average geologically older, more weathered with lower supply of P relative to N, consequently resulting in higher N:P ratios in roots in tropical latitudes. The variation not explained by climate and soil (Table 2) is likely in part due to biological factors, for example, plant species (such as differences in nutrient economy), stand age and disturbance history 12,33 .…”

“…Plant N and P stoichiometry is related to the variation among species, plant developmental stages, tissue types and environments 6,[9][10][11][12] . Plant green-leaf N and P concentrations have been shown to vary among climatic and soil environments, presumably reflecting both plant adjustment (acclimation and adaptation) 13 and biogeographical differences in relative N and P supply 14 .…”

Nitrogen response efficiency of a managed and phytodiverse temperate grassland

“…Bridgham et al (1995) gave a mathematical proof that nutrient response efficiency has to be unimodal across broad ranges of nutrient supply. However, the unimodal efficiency has not been observed for N so far and is debatable (Yuan et al 2006). A monotonic increase of NRE with decreasing soil N availability was observed by Yuan et al (2006) for Mongolian grassland.…”

“…However, a linear relationship over a wide range of N supply is unlikely given the observation that no further increase in productivity occurs at high levels of N availability (Pastor and Bridgham 1999). Studies that investigated the relationships between NRE and soil N availability found increases in NRE as soil N availability declined in an experimental plantation of tropical trees (Hiremath and Ewel 2001;Yuan et al 2006). Bridgham et al (1995) gave a mathematical proof that nutrient response efficiency has to be unimodal across broad ranges of nutrient supply.…”

Joint Control of Net Primary Productivity by Climate and Soil Nitrogen in the Forests of Eastern China

“…Significant advances have been made in the past decades toward understanding the relationship between soil N and NPP in terrestrial ecosystems, whereas substantial uncertainties persist, and discrepancies between studies remain unresolved [40][41][42]. Yuan et al (2006) [43] found a significant positive linear correlation between NPP and soil inorganic N for grassland ecosystems in Mongolia, whereas Luo et al (2004) [10] found a curved relationship between NPP and total soil N. Our field data indicated that N soil alone explained nearly 10.6% of the NPP variation within the transect ( Figure 4, Table 2). Hobbie (2015) [23] focused on feedback to NPP operating through litter decomposition, and reported that positive or negative effects of litter N on the later stages of litter decomposition could strengthen or weaken the positive loop of NPP.…”