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As global climate warms and atmospheric nitrogen deposition increases, plants are likely to respond by altering community composition, thus affecting the nutrient cycle. Although it is well-known that feedback between plants and soils is linked by nutrient resorption occurring during senescence, our understanding of how long-term warming and nitrogen deposition (> 10 years) influence nutrient resorption remains limited. In a desert grassland in northern China, we explored the effects of warming and nitrogen fertilization on leaf nutrient resorption for three dominant species during two hydrologically contrasting years (wet with 52% above the long-term mean precipitation of 222 mm, and dry with precipitation 16% below the mean), based on a manipulative experiment initiated in 2006. In the wet year, both warming and nitrogen fertilization significantly increased nitrogen and phosphorus concentrations in soils and plants and thus decreased resorption efficiency of both nitrogen and phosphorus with significant interactions, but these effects did not occur in the dry year. Changes in resorption efficiency were associated with plant available nitrogen and phosphorus in soils and water availability. Our study suggests that the responses of nutrient resorption to warming and nitrogen fertilization could be modified by natural precipitation variations in a desert grassland that is sensitive to abrupt changes in weather patterns.

Precipitation and nitrogen (N) deposition are predicted to increase in northern China. The present paper aimed to better understand how different dominant species in semi-arid grasslands in this region vary in their litter decomposition and nutrient release responses to increases in precipitation and N deposition. Above-ground litter of three dominant species (two grasses, Agropyron cristatum and Stipa krylovii, and one forb, Artemisia frigida) was collected from areas without experimental treatments in a semi-arid grassland in Inner Mongolia. Litter decomposition was studied over three years to determine the effects of water and N addition on litter decomposition rate and nutrient dynamics. Litter mass loss and nutrient release were faster for the forb species than for the two grasses during decomposition. Both water and N addition increased litter mass loss of the grass A. cristatum, while the treatments showed no impacts on that of the forb A. frigida. Supplemental N had time-dependent, positive effects on litter mass loss of the grass S. krylovii. During the three-year decomposition study, the release of N from litter was inhibited by N addition for the three species, and it was promoted by water addition for the two grasses. Across all treatments, N and potassium (K) were released from the litter of all three species, whereas calcium (Ca) was accumulated. Phosphorus (P) and magnesium (Mg) were released from the forb litter but accumulated in the grass litter after three years of decomposition. Our findings revealed that the litter decomposition response to water and N supplementation differed among dominant plant species in a semi-arid grassland, indicating that changes in dominant plant species induced by projected increases in precipitation and N deposition are likely to affect litter decomposition, nutrient cycling, and further biogeochemical cycles in this grassland. The asynchronous nutrient release of different species’ litter found in the present study highlights the complexity of nutrient replenishment from litter decomposition in the temperate steppe under scenarios of enhancing precipitation and N deposition.

Soil organic nitrogen (N) turnover is significantly influenced by elevated N deposition, precipitation and human-caused disturbances, but the underlying mechanism remains unclear. Identifying the relationships among the soil organic N fractions and N-mineralizing enzymes activities may advance our knowledge of the dynamics of soil organic N. A field experiment was conducted in a semi-arid steppe and an abandoned cropland in northern China to investigate the effects of elevated N deposition and precipitation on soil organic N fractions and their relationships with N-mineralizing enzymes, i.e., protease, amidase, urease and N-acetyl-β-D-glucosaminidase (NAG) activities. The concentrations of N in various fractions were consistently lower in the abandoned cropland compared with the steppe. Nitrogen addition consistently decreased amino acid N content and activities of urease, protease and amidase in both sites but increased amino sugar N content and NAG activity in the steppe. Water addition decreased hydrolysable ammonium N content but increased amino sugar N content and activities of protease and NAG in both sites. Furthermore, urease and NAG activities were significantly positively correlated with the proportions of amino acid N and amino sugar N and, explained significant proportions of the variations in soil organic N fractions in the steppe. However, soil organic carbon (C), rather than N-mineralizing enzymes, explained greatest proportion of the variations in soil organic N fractions in the abandoned cropland. The concurrent increase of N deposition and precipitation could promote the recovery of soil N (and C) losses in the abandoned cropland resulting from previous agriculture. Furthermore, in the steppe where NH4 + was available at relative high concentrations, enzymatic mineralization was the dominant route involved in potential soil organic N turnover. However, the direct route may be favored over the enzymatic mineralization route with decreasing availability of C relative to N in the abandoned cropland, which is driven by the need for C. These findings confirmed that the forms of N available, and the relative availability of C and N determine N uptake pathways both through enzymatic mineralization route and direct uptake route in the semi-arid grasslands.

Evidence for impacts of environmental changes on belowground net primary production (BNPP) from long-term experiments is rather scarce. We aimed to understand how long-term changes in water and nitrogen availability affect production and vertical allocation of roots in semi-arid grasslands and its consequence on carbon (C) and nitrogen (N) cycles. We investigated changes of BNPP and its vertical allocation along the soil profile to 40 cm in depth in response to simultaneous increases in water and N availability over 11 years in an old-field grassland in northern China. Water addition increased BNPP in all soil layers (0–10 cm, 10–20 cm, and 20–40 cm), and enhanced the percentage BNPP in the upper soils but decreased that in 10–20 cm soil layer. Nitrogen addition decreased BNPP in 10–20 cm and 20–40 cm soil layers as well as total BNPP in 0–40 cm, and increased the percentage BNPP in the upper soil layer but decreased that in 10–20 cm soil layer. Water addition increased soil total C and N concentrations in 0–10 cm and 10–20 cm soil layers, while N addition only marginally decreased soil C:N ratio in 0–10 cm and 20–40 cm soil layers. Both soil total N concentration and soil C:N ratio were closely related to BNPP. Our results highlight the importance of environmental factors, especially water availability, in determining BNPP, and in turn controlling soil nutrient accumulation in semi-arid grasslands, although the specific mechanisms remain unclear. The projected increase in precipitation in those semi-arid grasslands would enhance soil C and N sequestration. The increased allocation of BNPP in upper soils with long-term precipitation increment and N deposition may accelerate the cycles of C and N in these ecosystems, and thus increase the risk of soil C and N loss.

Theoretical and observational studies have suggested that environmental variations would change compositional similarity between plant communities. However, this topic has rarely been examined via experiments involving direct manipulation of resources utilized by plant communities. A 9-year field manipulation experiment was conducted to examine the effects of nitrogen addition and increased water on community similarity between a steppe and an old field in the semiarid region of northern China. Over the experimental period, nitrogen addition reduced community similarity between the steppe and the old field, whereas water addition enhanced community similarity. These treatment effects were closely related to changes in diversity characteristics as well as abundance of functional groups and dominant species of plant communities. These results highlight the importance of resource availability in regulating the trajectory of ecosystem succession, and suggest that the increase in atmospheric nitrogen deposition in northern China will contribute to divergence between the steppe and the old field, whereas the increase in growing-season precipitation may encourage convergence between the two grasslands with respect to species composition during succession. Thus the decrease in community similarity caused by nitrogen enrichment may be counteracted, at least partially, by precipitation increase under changing atmosphere and climate.

Background and Aims Given that plant growth is often water limited in drylands, it has been proposed that water seems to influence productivity by altering physiological/ metabolic responses and nutrient availability in short term. It is unclear, however, whether water mediates a positive plant-soil feedback and whether the feedback drives variations in plant productivity. Methods A 4-year field experiment was performed to examine the effects of water and nitrogen (N) addition on nutrient concentrations in soil and plant, nutrient resorption and potential return, in a temperate grassland in northern China. Results Water addition enhanced plant N and phosphorus (P) concentrations but reduced plant N and P resorption efficiency, leading to the increased potential N and P return to soil via litterfall. Enhanced nutrient potential return likely contributed to an increase of plant productivity in the following year. These “fertilization effects” caused by water addition were similar to those by N addition. Conclusions Our study suggests that the positive plantsoil feedback induced by increased precipitation may have a role in water-induced increases in productivity, and highlights the “fertilization effect” of water addition in a semiarid grassland in short term.

Previous studies have demonstrated positive net primary production effects with increased nitrogen (N) and water availability in Inner Mongolian semi-arid grasslands. However, the responses of soil carbon (C) and N concentrations and soil enzyme activities as indicators of impacts of long-term N (urea) and water addition are still unclear. We tested the effect of 7 years of a N and water addition experiment on soil C, N, and specific soil-bound enzymes in a semi-arid grassland of Inner Mongolia. We determined concentrations of soil organic carbon (SOC) and soil total nitrogen (TN) in both the 0–10 and 10–20 cm soil layers. Concentrations of labile carbon (LC) and inorganic nitrogen (nitrate and ammonium), and soil pH were measured. Additionally, soil dehydrogenase (DHA), β-glucosidase (BG) and acid and alkaline phosphomonoesterase (PME) enzyme activities were determined in the 0–10 cm soil layer. SOC concentration in the 0–10 cm soil layer showed no response to N addition or N plus water addition, but increased with water addition alone by 0.3–15.7 %. N addition significantly increased nitrate by 46.0–138.4 % and ammonium by 19.0–73.3 % in the 0–10 cm soil layer, whereas water addition did not affect them. The activities of DHA and alkaline PME enzymes, as well as soil pH, in the 0–10 cm layer decreased with N addition, however water addition alone caused these enzyme activities to increase. Unlike the surface soil (0–10 cm), the lower soil layer (10–20 cm), was responsive to N and water addition in that SOC and TN concentrations decreased with N addition and increased with water addition. The accumulation of SOC and TN in N and water addition plots may be caused by the input of plant biomass exceeding SOC decomposition. Decrease in microbial activity, derived from decreased DHA and alkaline PME activities might result from suppression effects of lower pH and decreased microbial N supply. Water availability is proved to be more important than N availability for soil C and N accumulation in this semi-arid grassland.