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For decades, arid desert ecosystems in northwest China, covering one-fourth the country's land surface, have experienced a rapid decline in plant species diversity, productivity and soil carbon stock owing to degradation by overgrazing. In this study, plant community composition, diversity and productivity, as well as soil carbon (C) and nitrogen (N) stocks, were monitored over 26 years from 1981 to 2006 in a severely degraded Haloxylon ammodendron-dominated shrubland where livestock densities were reduced from 4-5 to 1-2 dry sheep equivalent ha -1 . The objective was to assess long-term grazing effects on vegetation and soil C and N accumulation dynamics. Results showed that the reduction of grazing pressure significantly increased vegetation cover, plant diversity and productivity, resulting primarily from an increase in livestock-preferred species. Controlled grazing also led to marked increases in soil C and N stocks in the top 30 cm of soil. This increase was strongly associated with increased plant species richness, vegetation cover and biomass production. Averaged over 26 years, soil C and N accumulated at rates of 89.9 gC and 8.4 gN m -2 year -1 , respectively, but rates of C and N accumulation varied greatly at different time periods. The greatest species regeneration occurred in the first 8 years, but the largest C and N accumulation took place during years 9-18, with a time-lag in response to changes in vegetation. Our results provide insights into the long-term recovery patterns of different ecosystem components from the influence of prolonged overgrazing disturbance that cannot be inferred from a short-term study. The findings are important for assessing the resilience of these livestock-disturbed desert ecosystems and developing a more effective strategy for the management of this important biome from a long-term perspective.

Monitoring of drinking water has shown an increase in nitrate-nitrogen (NO 3 − -N) concentration in groundwater in some areas of the Heihe River Basin, Northwest China. A combination of careful irrigation and nitrogen (N) management is needed to improve N uptake efficiency and to minimize fertilizer N loss. A 2-year experiment investigated the effects of different irrigation and N application rates on soil NO 3 − -N distribution and fertilizer N loss, wheat grain yield and N uptake on recently reclaimed sandy farmland. The experiment followed a completely randomized split-plot design, taking flood irrigation (0.6, 0.8 and 1.0 of the estimated evapotranspiration) as main plot treatment and N-supply as split-plot treatment (with five levels of 0, 79, 140, 221, 300 kgN ha −1 ). Fertilizer N loss was calculated according to N balance equation. Our results showed that, under deficit irrigation conditions, N fertilizer application at a rate of 300 kgha −1 promoted NO 3 − -N concentration in 0-200 cm depth soil profiles, and treatments with 221 kgN ha −1 also increased soil NO 3 − -N concentrations only in the surface layers. Fertilizer N rates of 70 and 140 kgha −1 did not increase NO 3 − -N concentration in the 0-200 cm soil profile remaining after the spring wheat growing season. The amount of residual NO 3 − -N in soil profiles decreased with the amount of Plant Soil (2010) 337:325-339irrigation. Compared with N 0 , the increases of fertilizer N loss, in N 79 , N 140 , N 221 and N 300 respectively, were 59.9, 104.6, 143.5 and 210.6 kg ha −1 over 2 years. Under these experimental conditions, a N rate of 221 kgha −1 obtained the highest values of grain yield (2775 kgha −1 ), above-ground dry matter (5310 kg ha −1 ) and plant N uptake (103.8 kgha −1 ) over 2 years. The results clearly showed that the relative high grain yield and irrigation water productivity, and relative low N loss were achieved with application of 221 kgN ha −1 and low irrigation, the recommendation should be for those farmers who use the upper range of the recommended 150-400 kgN ha −1 , that they can save about 45% of their N and 40% of their irrigation water application.

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