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Abstract: Nitrogen (N) and precipitation are the main limiting factors in food production under rain-fed cropping systems in arid and semiarid regions. A long-term field experiment was conducted from 1984 to 2009 in the Changwu State Key Agro-Ecological Experimental Station of the Loess Plateau, China, to study interactions between precipitation and N fertilization. Winter wheat (Triticum aestivum L.) was grown with N fertilization at 0, 45, 90, 135, and 180kgNha−1. With 25-year N fertilization, soil organic carbon and total N were increased by 18% and 26%, respectively. Mean yields and precipitation use efficiency (PUE) levels with N fertilization were increased by onefold to twofold compared with unfertilized soils. Generally, during the period of 1984–2009, grain yields with 45–`180kgN ha−1 increased at the rate of 38–49kgha−1 year−1 (p <0.05), whereas PUE increased at the rate of 0.071–0.088kgmm−1 year−1. These increases over time are largely attributable to improvements in soil organic carbon and total N. Variations in yield were proportionally reduced by N fertilization, but heavily depended on the fluctuation of precipitation. Grain yields with 45–180kgN ha−1 decreased by approximately 10%–45% (p <0.05) in the dry years (drought index<−0.35) compared with the normal years, whereas significant increases were not detected in the wet years (drought index>0.35). Grain yields with 90–180kgN ha−1 were positively correlated (p <0.05) with fallow season precipitation, but insignificantly correlated with growing season precipitation. The optimum N fertilization rates in the dry, normal and wet years increased following an increasing sequence of 45, 135 and 180kgNha−1. These results have significant implications for optimizing N fertilization and maximizing yield and PUE in rain-fed cropping systems in arid and semiarid regions. [Copyright &y& Elsevier]

Nitrogen (N) fertilization has been repeatedly reported to strongly influence soil properties and crop growth. However, there is little information about the combined effects of soils and crops on soil CO 2 fluxes under N fertilization. In this study, in situ soil respiration, soil physicochemical properties, microbial communities and crop properties were measured for eight years (2008 to 2016) on the Loess Plateau. Five rates of N fertilization were applied to different plots to compare the soil respiration rate and its temperature sensitivity (Q 10). Nitrogen fertilization significantly increased mean annual cumulative soil respiration (R cum) by 25%–44%. R cum had a positive correlation with grain yield, and the carbon emission efficiency (grain yield produced per unit of carbon emission) under N-fertilized plots was 1.62–2.52 times that of unfertilized plots. R cum also had a positive correlation with root biomass and the root N concentration but showed a negative correlation with the root C/N ratio. The Q 10 values under N-fertilized plots decreased by half at a diurnal scale, but had a smaller reduction (i.e. , 0.04–0.09) at an annual scale compared to unfertilized plots. The decreased Q 10 values under N-fertilized plots also resulted from the lower aromaticity of dissolved carbon (SUVA 254) (7.40 vs. 10.53 L mg C−1 m−1). In addition, the altered R cum and Q 10 values were affected by the varied bacteria community derived from N fertilization, which was related to Acidobacteria , Chloroflexi , Proteobacteria and Bacteroidetes. Therefore, N fertilizer applications regulate the combined effects of soil and crop parameters on soil respiration and the Q 10 value. This study suggests that, due to the lower carbon emission efficiency and higher SOC concentration under N-fertilized plots, N fertilizer applications may be used to sustain crop yields and increasing SOC storage while minimizing carbon emission impacts to the environment on the Loess Plateau. • Nitrogen fertilizer application increased soil respiration rate (SR) but decreased Q 10. • Variation in SR and Q 10 was resulted from altered photosynthetic rate, C/N, SUVA 254 , and bacteria community. • Nitrogen fertilizer application increased carbon emission efficiency. [ABSTRACT FROM AUTHOR]

Temperature sensitivity of soil respiration ( Q 10 ) is an important parameter when modeling the effects of global warming on terrestrial ecosystem carbon release. Widely applied chemical fertilizers can significantly affect soil productivity and carbon cycling in agroecosystems. However, little is known about how Q 10 responds to chemical fertilization under different levels of initial soil fertility. On the Chinese Loess Plateau, changes in soil respiration rates and Q 10 were investigated in soils of two fertility levels: low fertility (L) and high fertility (H). For each soil fertility level, there was one control plot and one chemical fertilized plot (+NP), which in total formed four treatments: L, L + NP, H and H + NP. All the treatments were replicated for three times on a continuous winter wheat cropping system. Respiration rates of surface soil in each treatment were in situ monitored from October 2010 through September 2015. Our results showed that after NP fertilization, soil respiration rates were increased by 46% in low fertility soil, yet only by 14% in high fertility soil ( P  < 0.05). The Q 10 after NP fertilization was significantly decreased by 6.9% in low fertility soil, but was unchanged in the high fertility soil. The Q 10 variation might be attributed to the different response of microbial respiration Q 10 in the two soils. The decreased Q 10 with NP fertilization in the low fertility soil was possibly due to N-induced increase of substrate quality for soil microbes and increased activities of both cellobiohydrolase and polyphenol oxidase. In the high fertility soil, the unchanged Q 10 with NP fertilization may be the integrated result of less affected substrate quality and neutral response of polyphenol oxidase activity. Overall, our results suggested that the effects of NP fertilization on soil respiration and its temperature sensitivity varied with soil initial fertility levels, and therefore must be properly accounted for when estimating potential effects of local agricultural management to regional agroecosystems under future climate conditions. [ABSTRACT FROM AUTHOR]

Many ecosystem functions and processes depend on biodiversity, however, the effect of root diversity in agroecosystems on soil bacterial communities and processes remained largely unknown. Our objectives were to examine the importance of increased root diversity through crop rotation on soil bacterial community composition and its relationship with soil carbon (C) and nitrogen (N) accumulation, which play an important role in soil fertility. In a field experiment with 30-year crop rotation, where there was no difference in root biomass input in top soil, soil C and N accumulation rates, soil microbial activities and bacterial community composition were investigated. Soil C and N accumulation rates and microbial biomass content were generally increased after rotation, with a greater increase in legume-cereal rotation than in cereal-cereal rotation. Crop rotation also increased soil microbial activity (soil respiration, potential N mineralization), but did not affect soil bacterial diversity. The increased bacterial abundance and changes in bacterial community structure and abundances of dominant bacterial phyla in rotation soils were related to increases in soil C and N accumulation and microbial activity. Our results suggest that increased root diversity through rotation can influence soil bacterial community structure and increase soil fertility by enhancing C and N accumulation rates, and cause positive effects on soil organic C and fertility. The influence of legume-cereal rotation was greater than that of cereal-cereal rotation. [ABSTRACT FROM AUTHOR]

• Topsoil removal decreased C sequestration in soil-plant ecosystems. • Topsoil addition increased C sequestration in soil-plant ecosystems. • Soil redistribution reduced integrated C sequestration in soil-plant ecosystems. Soil redistribution is an important movement process of surface material that drastically affects the dynamic cycling of essential elements and soil productivity in the terrestrial ecosystem. Although a number of studies have focused on the soil organic carbon (SOC) pool under soil redistribution, few studies have comprehensively assessed the effects of soil redistribution on the ecosystem carbon pool. A five-year experiment of simulated topsoil redistribution (removal and addition, R-A) was conducted to explore the effects of soil redistribution on soil CO 2 emissions, the SOC pool and the plant C pool in the semi-arid Loess Plateau of China. The soil properties, soil CO 2 emissions, and crop parameters were measured after the topsoil removal (R 20) treatment and topsoil addition (A 20) treatment and in the undisturbed control (CK). Our results showed that 1) compared with the undisturbed control, the mean soil CO 2 emissions of the R-A treatment decreased by 8% while the mean SOC pool increased by 3.0%; 2) the plant C pool was significantly reduced by 53.8% in 2015 and 5.8% in 2019 compared with the undisturbed control; 3) the ecosystem C pool (the sum of the SOC pool and plant C pool) of the R-A treatment was reduced by 10.3%, 3.7% and 1.1% compared with the ecosystem C pool of the undisturbed CK for 2015, 2016 and 2017, respectively, but increased by 0.6% and 1.9% from 2018 to 2019. During the first three experimental years, the decrease in the ecosystem C pool of the R-A treatment was caused by the reduced plant C pool in R 20 exceeding the elevated plant C pool in A 20. Over time, the soil bulk density (BD) decreased in R 20 and the SOC, microbial biomass carbon (SMBC) and total nitrogen (TN) increased, which jointly improved the plant C pool in R 20 and the ecosystem C pool by 2019. The gradual recovering pattern of the ecosystem C pool in the later years highlights the key role of plants in restoring soil properties and stabilizing ecosystem C cycling. Overall, our findings highlight that any attempt to assess the net impacts of soil redistribution, whether as a sink or source for atmospheric CO 2 , should comprehensively consider the changes in the SOC pool and plant C pool. [ABSTRACT FROM AUTHOR]

• Bacterial alpha diversity was increased by deposition but not changed by erosion. • Erosion altered bacterial community structure but deposition did not. • Bacterial co-occurrence network was more complex at depositional than eroding site. Soil erosion and deposition occur widely from regional to global scales and have profound impacts on ecological services and sustainability. Despite their crucial roles in biogeochemical cycles, the responses of soil bacterial communities to soil erosion and deposition remain largely unclear. In this study, a field simulation experiment was conducted to examine variation in soil bacterial communities across eroding slopes and depositional zones with three slope gradients (5°, 10° and 20°) on the Loess Plateau of China (2015–2017). The results showed that soil physicochemical properties were altered by redistribution of runoff and sediment across eroding slopes and depositional zones. Soil bacterial alpha diversity was higher in the depositional zones of both 10° and 20° slopes compared with the 5° reference slopes but no markedly difference was found between eroding slopes and reference slopes. By contrast, bacterial community structure differed between eroding slopes and reference slopes but not between depositional zones and reference slopes. Differentiation of bacterial communities between eroding slopes and depositional zones increased with increasing slope gradients. The bacterial network was greater and more complex within depositional zone than eroding slope, indicating more extensive bacterial interactions and greater community stability potential. Erosion- and deposition-induced redistribution of soil moisture, soil organic matter, available P, and available K were the key determinants of variation in bacterial community structure. Our findings demonstrate the contrasting effects of soil erosion and deposition on soil bacterial communities, which should be given further attention across eroding landscapes. [ABSTRACT FROM AUTHOR]