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Topography has strong effects on microclimates; thus may influence the decomposition of organic matter, a key process determines soil nutrient availability and carbon fluxes in terrestrial ecosystems. Yet, little is known if and how topographic factors influence litter decomposition. We studied the effects of slope aspect (south- vs. north-facing slopes) and position (base and middle positions) on plant shoot litter and root decomposition. We analyzed dynamics of litter mass loss in a 442-day period, and soil and vegetation characteristics in a typical semiarid hilly grassland. Our results showed that the decomposition of roots was faster at south-facing than at north-facing sites, which can be explained by the 2 °C higher soil temperature at south-facing sites. Decomposition rate of shoot litter were not different between slope aspects. North-facing sites had 76% higher aboveground biomass and 80% higher belowground biomass than those at south-facing sites. Accordingly, plant N and C storages at north-facing sites were 67% and 76% higher than those at south-facing sites, respectively. The smaller plant carbon and nitrogen stocks and the faster root decomposition at south-facing slopes suggest higher proportions of plant C and N are lost from the ecosystem than that at north-facing slopes. This work highlights the necessity of taking slope aspect into account in carbon and nitrogen cycling studies in hilly grasslands. [ABSTRACT FROM AUTHOR]

Herbivory and resource availability interactively regulate plant growth, biomass allocation, and production. However, the compensatory growth of plants under grazing intensities and manipulated environmental conditions is not well understood. A 2-year experiment with water (unirrigated and irrigated) and nitrogen fertilizer (0 and 75 kg N ha −1 year −1 ) addition was conducted at sites with 4 grazing intensities (0–7 sheep ha −1 ) in an annually rotational grazing system in Inner Mongolia. In this study, grazing had no significant effect on aboveground net primary production (ANPP) and net primary production (NPP). However, high grazing intensity strongly reduced the fraction of belowground net primary production to NPP. Water and nitrogen additions significantly increased ANPP by 39% and by 23%, respectively, but had no effect on belowground net primary production. ANPP showed lower response to nitrogen addition at high grazing intensity sites than at low grazing intensity sites. We found no evidence for grazing optimization on primary production of semiarid steppe, regardless of resource supplementations. Grazed plants minimized the reduction of ANPP by altering allocation priority and morphological traits. Our study highlights the “whole-plant” perspective when studying plant–herbivore interactions. [ABSTRACT FROM AUTHOR]

Abstract: Question: Climate change interacts with land use and introduces new pressures that trigger growing concerns about increasing vulnerability of the Eurasian steppes. However, it is not well known how increasing precipitation and atmospheric N deposition interact with the land use legacy to affect nutrient availability, plant species composition and therefore vegetation dynamics of the Mongolian Steppe. Location: Steppe in Xilin River Basin, Inner Mongolia, China. The mean annual precipitation is 343 mm with 60%–80% of it occurring during the plant growing season. Methods: We conducted a 6‐years (2005–2010) field experiment to manipulate N and water availability on sites experiencing two different historical stocking rates. Species composition, above‐ground biomass and plant N concentration were determined at both individual and community levels. Soil cumulative inorganic N and N mineralization rates were determined by laboratory incubation. Results: (1) Supplementary irrigation increased soil cumulative inorganic N and N mineralization rate, plant community N uptake, and the abundance of perennial species for the site with high historical stocking rate. In contrast, long‐term water addition decreased soil cumulative inorganic N and N mineralization rate, and did not change the plant community N uptake, but increased the abundance of Cleistogenes squarrosa as a species indicative of degradation for sites with moderate historic stocking rate. (2) Nitrogen addition increased soil cumulative inorganic N and N mineralization rate irrespective of grazing history under ambient precipitation, and resulted in a burst of annuals in moist years at both sites. Under supplementary irrigation, N addition increased soil cumulative inorganic N and N mineralization rate at the site with moderate historic stocking rate, but not at the site with high historical stocking rate, and increased the abundance of taller perennial species at both sites. Conclusions: Legacy effects of grazing greatly affect plant composition responses to increasing water and N availability. The effectiveness of N application in restoring heavily degraded sites in the Mongolian steppe depends on precipitation availability. In the face of increasing precipitation and atmospheric N deposition, resource managers should be prepared to cope with the different vegetation succession and recovery trajectories as a result of historical land‐use difference. [ABSTRACT FROM AUTHOR]

Grazing removal is widely used in grassland management. Plant responses following grazing removal at different organizational levels, however, are not well understood. We examined plant responses at different stocking rates in an Inner Mongolia grassland ecosystem dominated by Leymus chinensis and Stipa grandis. Our results indicated that plant response patterns differed significantly among stocking rates, at different levels of organization, and between wet and dry years. Community aboveground net primary production (ANPP) recovered more quickly at low and moderate stocking rates than those at high stocking rates. Response of aboveground net primary production (R) was significantly positively correlated with both individual biomass and density responses of L. chinensis. Overcompensation of L. chinensis after grazing removal contributed greatly to positive R at the community level. Significant compensatory effects were found between the two dominant species and between dominant species and the remaining non-dominant species. Variation in precipitation significantly affected community ANPP, relationships between community and species responses, and compensatory effects between species. Our study suggests that periodic grazing removal is likely to be a useful method for grassland management and that a combination of species with compensatory effects can be advantageous for reseeding practices in grassland restoration. [ABSTRACT FROM AUTHOR]

In water-limited environments, photosynthetic carbon gain and loss of water by transpiration are in a permanent tradeoff as both are contrarily regulated by stomata conductance. In semiarid steppe grasslands water limitation may covary with nitrogen limitation. Steppe grassland species are capable of optimizing their use of limiting resources, water and nitrogen, but regulation is still poorly understood. In a two-year experiment with addition of water (irrigation simulating a wet year) and nitrogen (0, 25, and 50 kg urea-N ha) we assessed intrinsic water use efficiency (WUE), nitrogen use efficiency (NUE), and related plant functional traits (PFTs) of four dominant C species ( Leymus chinensis, Agropyron cristatum, Stipa grandis, and Artemisia frigida). Water and N fertilizer supplementation significantly increased plant primary production, and N effect was more pronounced under irrigated conditions. Parallel with the responses of plant production, a strong tradeoff between WUE and NUE was detected: water supply increased NUE but decreased WUE, whereas N addition slightly increased WUE at the expense of NUE. This tradeoff occurred at the leaf level, and involved the responses of leaf N concentration and specific leaf area. WUE of species changed among treatments in a predictable manner by the parameter of leaf N content per area. Dominant plant species commonly achieved a higher utilization efficiency of the more limiting resource via altering PFTs, which was an important mechanism of adaptation to variable resource limitation in semiarid grasslands. [ABSTRACT FROM AUTHOR]

Inner Mongolia steppe grasslands are widely used for livestock farming and the regrowth ability of grassland species is therefore strongly influenced not only by water and nutrient availability but also quite heavily by grazing. However, little is known on how grazing, water and nitrogen interactively affect the dominant C3 species ( Leymus chinensis, Stipa grandis, Agropyron cristatum) and the C4 species ( Cleistogenes squarrosa). Therefore in the 2007 and 2008 growing seasons, a field experiment was carried out to test the hypothesis that under different grazing intensities the dominant species show different short-term regrowth response to simultaneous variation in the availability of water and nitrogen. Single factor and interaction effects of the addition of water (rainfed vs. simulated wet-year) and nitrogen (0 or 25 kg N ha) were analysed along a gradient of four grazing intensities (ungrazed, lightly, moderately and heavily grazed) after one month of grazing exclusion. Water and nitrogen addition affected short-term regrowth of all species in a similar way whereas species responded differently to grazing. Simulated wet-year water availability consistently resulted in higher standing biomass, relative growth rate and cellulase digestible organic matter yield. Supplementary nitrogen promoted standing biomass and crude protein concentration. The nutritive value of all species' standing biomass showed a similar increase with more intensive grazing. However, heavy grazing led to a clear shift in the relative biomass of the species, i.e. mainly a promotion of the C4 grass, C. squarrosa. In contrast to our hypothesis, there were no differences among species in their response to water or nitrogen addition, whereas, heavy grazing induced the expected species-specific response. Our results suggest that heavy grazing rather than nitrogen or water determine short-term shifts in species composition of the investigated steppe ecosystem. Furthermore, differences in the species-specific growth response to grazing may increase the proportion of the C4 grass C. squarrosa in steppe communities, whereas higher availability of nitrogen and water may lead to higher forage biomass and nutritive value of all investigated species but in short-term cannot compensate for the grazing induced changes in species composition. [ABSTRACT FROM AUTHOR]