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Background and aims: the nitrogen isotope composition (δ¹⁵N) of plants in arid and semiarid grasslands is affected by environmental factors, especially water availability. Nevertheless, it is unclear whether the response of δ¹⁵N to water availability differs between C₃ and C₄ photosynthetic pathways. - Methods: we investigated plant δ¹⁵N of coexisting C₃ and C4 species as a function of aridity along a 3200 km aridity gradient across the arid and semi-arid grasslands of northern China. - Results: aridity was positively correlated with plant δ15N values in both C₃ and C₄ plants and also in the entire plant community, whereas soil bulk δ¹⁵N values increased first and then decreased along the aridity gradient. The N uptake by C₄ plants appeared to be more affected by competition pressure of neighboring plants and soil microbes than for C₃ plants along the transect. - Conclusions: the decoupled relationship between plant and soil δ15N values indicated that variations in vegetation and soil δ15N values were driven by differential biogeochemical processes, while different soil N sources were used for plant growth along the climatic gradient. The advantage of C₃ plants in the use of N may counteract the competitive advantage that C₄ plants have over C₃ plants due to their higher water use efficiency under drier conditions. These findings can help understand why C₄ plants do not completely replace C₃ plants in drier environments despite their higher water use efficiency.

Break crops are used in agriculture to reduce soil pathogens and improve yield of subsequent cereal crops. However, they can also affect soil microbial communities beneficial to plant growth including arbuscular mycorrhizal fungi (AMF). Two wheat genotypes (IAW2013 and 249) were planted after crop rotation with canola or chickpea with different nitrogen (N) and phosphorus (P) fertilizer treatments (0 and 100 kg N ha(-1) and 0 or 20 kg P ha(-1)) in the field. Plant and soil available N and P, AMF root colonization, shoot biomass, wheat yield and leaf delta C-13 were examined. While crop rotation did not affect soil available N and P, AMF colonization in wheat was on average 60% higher after chickpea than after the canola rotation. Wheat yield after chickpea increased for genotype IAW2013, and was positively related to AMF colonization for both genotypes. N and P fertilization reduced AMF colonization and yield, but increased shoot biomass and leaf tissue N and P concentrations. Leaf delta C-13 decreased with increased yield, suggesting that higher yielding and AMF colonized plants were less water stressed. In contrast to fertilization, cultivation of certain crops in the previous season, in our case chickpea, can promote AMF colonization of wheat roots, thereby increasing grain yield.

Drought is predicted to increase in many areas of the world, which can greatly influence soil microbial community structure and C stabilization. Increasing soil carbon (C) stabilization is an important strategy to mitigate climate change effects, but the underlying processes promoting C stabilization are still unclear. Microbes are an important contributor of C stabilization through the adsorption of microbial derived compounds on organo-mineral complexes. Management practices, such as addition of organic amendments might increase soil C stock and mitigate drought impacts, especially in agro-ecosystems where large losses of C have been reported. Here, we conducted a drought experiment where we tested whether the addition of organic amendments mitigates drought effects on soil C stabilization and its links to microbial community changes. In a semi-natural grassland system of eastern Australia, we combined a management treatment (compost vs. inorganic fertilizer addition) and a drought treatment using rainout shelters (half vs. ambient precipitation). We measured soil moisture, soil nitrogen and phosphorus, particulate organic C (Porn-C) and organo-mineral C (Min-C). Microbial community composition and biomass were assessed with PLFA analyses. A structural equation modeling (SEM) approach was used to examine the controls of soil moisture, Porn-C and nutrients on soil microbial biomass and community structure and changes in Min-C. Overall, the drought treatment did not affect microbial community structure and Min-C, while fertilizer only marginally increased Min-C, highlighting the resistance to these treatments in this grassland soil. In the surface soil (0-5 cm) Min-C was strongly associated with fungi that may have been stimulated by root exudates, and by gram-negative bacteria in the deep soil (5-15 cm) that were more affected by Porn-C and soil moisture. We conclude that the grassland microbial community and its effect on Min-C at our field-site were non-responsive to our drought treatment, but sensitive to variability in soil moisture and microbial community structure. Our findings also show that surface compost application can moderately increase soil C stabilization under drought, representing a useful tool for improving soil C stability. (C) 2016 Elsevier Ltd. All rights reserved.

1. Drought induces changes in the nitrogen (N) and phosphorus (P) cycle but most plant species have limited flexibility to take up nutrients under such variable or unbalanced N and P availability. Both the degree of flexibility in plant N:P ratio and of root symbiosis with arbuscular mycorrhizal fungi might control plant resistance to drought-induced changes in nutrient availability, but this has not been directly tested. 2. Here, we examined the role of plant N:P stoichiometric status and mycorrhizal symbiosis in the drought-resistance of dominant and subordinate species in a semi-natural grassland. 3. We reduced water availability using rainout shelters (control vs. drought) and measured how plant biomass responded for the dominant and subordinate species. We then selected a dominant (Paspalum dilatatum) and a subordinate species (Cynodon dactylon), for which we investigated the N:P stoichiometric status, mycorrhizal root colonization and water-use efficiency. 4. The biomass of all dominant plant species, but not subordinate species, decreased under drought. Drought increased soil available nitrogen, and thus increased soil N:P ratio, due to decreasing plant N uptake. The dominant P. dilatatum showed a high degree of plant N:P homeostasis and a considerable reduction in biomass under drought. At the opposite, the more flexible subordinate species C. dactylon increased its N uptake and water-use efficiency, apparently due to stronger symbiosis with mycorrhizae, and maintained its biomass. 5. Synthesis. We conclude that the maintenance of N:P homeostasis in dominant species, possibly because of a large root nutrient foraging capacity, becomes inefficient when water stress limits N mobility in the soil. By contrast, we demonstrate that higher stoichiometric N:P flexibility coupled with stronger mutualistic association with mycorrhizae allow subordinate species to better withstand drought perturbations. Using a stoichiometric approach in a field experiment, our study provides for the first time clear and novel understandings of the mechanisms involved in drought-resistance within the plant-mycorrhizae-soil system.