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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]

While water availability determines grassland productivity in semiarid regions, nutrient availability is the main limiting factor under wet conditions. An experiment was conducted in 2008 at two sites in Inner Mongolia with histories of heavy grazing (HG) and moderate grazing (MG) to study the interactive effects of water and nitrogen on above- and belowground net primary productivity (ANPP and BNPP), biomass partitioning, and plant species composition. The study comprises two water treatments (no irrigation and irrigated when soil water content was below 70% of the field capacity), and two nitrogen (N) levels (0 and 100 kg N ha). Mean values of ANPP at the peak biomass time reached 1,028 ± 95 SD g m at the HG site and 568 ± 32 SD g m at the MG site in irrigated and fertilized treatment. Nitrogen use efficiency (NUE) was significantly higher at irrigated plots compared to rain-fed plots at both HG and MG sites. Water use efficiency (WUEt) based on total water input and ANPP decreased with irrigation at the HG site. Meanwhile, N application significantly increased WUEt, WUEp (based on precipitation), and WUEi (based on irrigation water) at both sites. BNPP was significantly higher at irrigated plots compared to rain-fed plots at both HG and MG sites, and it tended to decrease with N addition. However, the fraction of belowground to total biomass ( f = BNPP/(ANPP+BNPP) decreased with the addition of supplemental resources and exhibited a negative correlation with ANPP. Species diversity remained lower at the HG site compared to the MG site; it decreased with the addition of supplemental resources at the latter site. The annual Salsola collina contributed the most to the total biomass under irrigation. Based on global climate models, more frequent extreme climates are predicted in the future, which can result in changes in resource availabilities. Therefore, our research results have important implications for predicting the production and other properties of grassland ecosystems. [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]

In grazed semiarid steppe ecosystems, much attention has been paid to aspects of growth limitation by water. So far, potential limitation of primary production by plant nutrients was rarely considered. This knowledge is essential for identification of sustainable land-use practices in these large and important ecosystems on the background of over-exploitation and climate change. In the present study plant nutrient concentrations and ratios were investigated with factorial additions of water and N fertilizer at two sites with contrasting soil nutrient availability. Combined analysis of nutrient concentrations, contents, biomass production, and plant N:P ratios consistently confirmed primary growth limitation by water and a strong N limitation when sufficient amounts of water were supplied. P limitation only occurred at the site with low P availability when in addition to the natural supply, water and N fertilizer were given. According to reported thresholds of N:K and K:P ratios, K was not limiting in any plot. The observed nutritional patterns in the plant community were related to the dynamics of species composition and their specific nutrient status. Stipa grandis had the highest N:P ratio whereas Artemisia frigida showed lowest N:P. These nutrient characteristics were related to growth strategies of dominant species. Accordingly, the relative biomass contribution of S. grandis and A. frigida strongly affected the nutrient status of the plant community. Plant N:P ratios indicate the relative limitation by N or P in the semiarid grasslands under sufficient water supply, but other methods of nutritional diagnosis should be used when plant N:P ratios remain below critical values. [ABSTRACT FROM AUTHOR]

Plant-microbe interactions are crucial regulators of belowground nitrogen cycling in terrestrial ecosystems. However, such interactions have mostly been excluded from experimental setups for the investigation of gross inorganic N fluxes and N partitioning to plants and microorganisms. Ungulate grazing is likely to feed back on soil N fluxes, and hence it is of special importance to simultaneously investigate grazing effects on both plant and microbial N fluxes in intact plant-soil systems, where plant-microbe interactions persist during the experimental incubation. Based on the homogenous NH labelling of intact plant-soil monoliths we investigated how various stocking rates (0, 2.35, 4.8 and 7.85 sheep ha grazing season) in steppe of Inner Mongolia feedback on gross rates of N mineralization and short-term inorganic N partitioning between plant, microbial and soil N pools. Our results showed that the effect of grazing on gross N mineralization was non-uniform. At low stocking rate gross N mineralization tended to decrease but increased with higher grazing pressure. Hence, there was no significant correlation between stocking rate and gross N mineralization across the investigated grazing intensities. Grazing decreased N recovery both in plant and microbial N pools but strongly promoted NO accumulation in the soil and thus negatively affected potential ecosystem N retention. This appeared to be closely related to the grazing-induced decline in easily degradable soil C availability at increasing stocking rate. [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]

The Fengqiu long-term field experiment was established to examine effects of organic manure and mineral fertilizers on soil total nitrogen (N) and natural 15N abundance. Fertilizer regimes include organic manure (OM), one-half N from organic manure plus one-half N from mineral N fertilizer (1/2OMN), mineral fertilizers [N-phosphorus (P)-potassium (K), NP, NK, PK], and a control. Organic manure (OM and 1/2OMN) significantly increased soil total N and δ15N, which was expected as a great amount of the N applied remained in soils. Mineral NPK fertilizer and mineral NP fertilizer significantly increased total N and slightly increaed δ15N. Phosphorus-deficient fertilization (NK) and N-deficient fertilization (PK) had no effect on soil total N. Significantly greater δ15N was observed in the NK treatment as compared to the control, suggesting that considerable N was lost by ammonia (NH3) voltalization and denitrification in this P-deficiency fertilization regime. [ABSTRACT FROM AUTHOR]

Abstract: In the system of “aerobic rice”, especially adapted aerobic rice varieties are grown under non-flooded conditions in non-puddled and aerobic soils with supplemental irrigation and moderate external inputs. Limited research has been done so far on optimizing nutrient management to produce high yields. In this study, we investigated yield formation and dry matter translocation of aerobic rice cultivar HD297 in response to N application, grown under different irrigation regimes at two sites close to Beijing, North China. At the “experimental farm”, the groundwater table depth was 0.2–1m; at the “experimental station”, it was deeper than 2m. Fertilizer-N rates were 0, 75, and 150kgNha−1 applied in split dressings according to regional recommendations for lowland rice, with 30% being applied at sowing, 40% at tillering, and 30% at panicle initiation. The application of N increased grain yields from 3.4 to 4.4tha−1. There was no interaction between N rates and water management, and no effect of water treatment on the grain yield. Before anthesis, dry matter accumulation increased with N application, but after anthesis, it decreased with increasing N application. With high N applications, grain filling was limited by a low contribution of post-anthesis assimilates. This indicates that traditional lowland rice fertilizer application schedules are not suitable for aerobic rice. N rate and timing of N fertilizer should be adjusted to balance crop''s demand before and after anthesis. Reducing N before anthesis and applying N at anthesis is necessary to increase post-anthesis dry matter accumulation and finally grain yield. Furthermore, germplasm improvement is suggested to increase the translocation of dry matter to grain for increasing grain yield. On average, yield was 14% higher at the “experimental farm” than at the “experimental station”. Differences in crop growth and Mn uptake between the farm and the station mainly resulted from the difference in hydrology. In target areas for aerobic rice with a deep groundwater table more research is needed to determine whether Mn deficiency is a potential cause of yield suppression of aerobic rice. [Copyright &y& Elsevier]