| The Loess Plateau has a typical semiarid monsoon climate, in which maize(Zea mays L.) is one of the most common grain crops; however, low air temperatures and drought during the early crop growth stage in the spring often result in poor crop yield. Currently, gravel and plastic film mulching, which have been widely used in the semiarid regions in China, may greatly improve crop yields due to increases in soil moisture and topsoil temperature. Alleviating the hydrothermal limitations to growth by mulching, however risks depleting the soil organic matter(SOM) and reducing soil quality through increased SOM mineralization and microbial activity. Therefore, keeping a satisfactory level of SOM is significant for ensuring food security and mitigating climate warming. Soil physical environment could be altered greatly by soil surface mulching, which would affect soil carbon and nitrogen cycling. In this study, based on a 5-year spring maize field experiment, we explored the responses of soil organic carbon and nitrogen as well as their labile fractions to different mulching practices, and aim to elucidate the long term changes in soil organic carbon and nitrogen pools as affected by soil surface mulching.The main conclusions were showed as follows:(1)Gravel and plastic film mulching had no effect on the soil organic C(SOC) content after 5 years of this study, while plastic film mulching increased the SOC stock by 2.0 Mg hm-2 across the 0-40 cm layer, compared with the initial SOC stock. Gravel and plastic film mulching significantly affect labile organic C fractions and carbon management index(CMI). Compared with the control, after 4 years of this study, gravel mulching significantly decreased light fraction organic C(LFOC) and water soluble organic C(WSOC) across the 0-40 cm layer and microbial biomass C(MBC) in the 20-40 cm layer, while plastic film mulching significantly decreased extractable organic C(EOC) in the 0-20 cm layer and significantly increased LFOC by 17.2 mg kg-1 in the 20-40 cm layer. Gravel mulching significantly decreased the mean CMI of 5 years in two layers, while plastic film mulching had no effect on the CMI after 5 years of this study. In conclusion, plastic film mulching could be the best option for sustainably enhancing crop productivity and maintaining soil quality in the semiarid regions of northwest China.(2)Gravel and plastic film mulching had no effect on the total N(TN) content and stock after 5 years of this study. Compared with the control, gravel and plastic film mulching significantly decreased light fraction organic N(LFON) by 12.2 and 6.5 mg kg-1, respectively, and extractable organic N(EON) by 7.7 and 9.3 mg kg-1 in the 0-20 cm layer; while significantly increased water soluble organic N(WSON) by 1.6 and 1.5 mg kg-1 in the 0-20 cm layer, respectively, and by 1.3 and 1.2 mg kg-1 in the 20-40 cm layer after 4 years of this study. Moreover, gravel mulching significantly decreased EON by 1.6 mg kg-1 in the 20-40 cm layer,plastic film mulching significantly increased the microbial biomass N(MBN) by 19.5 mg kg-1 in the 0-20 cm layer compared with the control after 4 years of this study. In general, plastic film mulching displayed greater effects on enhancing crop yield and increasing labile soil organic N(SON) pools than gravel mulching. It is suitable to choose plastic film mulching over gravel mulching to have increases in crop yields and improvements in soil N availability in the semiarid regions of northwest China.(3)Compared with traditional plastic film mulching during all growing stages(PM), plastic film removal at the silking stage(RM) caused soil moisture and temperature changes, which would affect labile soil organic carbon and nitrogen fractions as well as the CMI. The RM treatment had no effect on the SOC and TN content after 6 years of this study. Compared with the PM treatment, the RM treatment significantly increased the LFOC, LFON and MBC content by 0.05 g kg-1, 3.4 mg kg-1 and 14.7 mg kg-1 in the 20-40 cm layer, respectively, while significantly decreased the LFOC, LFON and MBN content by 0.24 g kg-1, 20.0 mg kg-1 and 26.1 mg kg-1 in the 0-20 cm layer after 5 years of this study. The RM treatment had no effect on the WSOC and WSON content,while significantly increased the EOC, EON and KMn O4-oxidizable C by 10.3 mg kg-1, 19.5 mg kg-1 and 0.59 g kg-1 in the 0-20 cm layer, respectively, and by 2.6 mg kg-1, 4.6 mg kg-1 and 0.10 g kg-1 in the 20-40 cm layer after 5 years of this study. The RM treatment also significantly increased the CMI in two layers. Therefore, plastic film removal is an effective option for increasing crop yields and maintaining soil sustainability in the regions with semiarid monsoon climates that have sufficient rainfall during the maize reproductive stages.(4)Soil surface mulching and plastic film-removed after the silking stage had significant effects on soil organic nitrogen mineralization. Using an in situ soil N mineralization device, this study was displayed in two years of the maize growing season and fallow season. Results showed that gravel mulching significantly increased the annual total N mineralization by 7.5 kg N hm-2 across the 0-40 cm layer, while plastic film mulching significantly decreased the annual total N mineralization by 7.1 kg N hm-2 across the 0-40 cm layer, compared with the non-mulched treatment. Compared with the plastic film mulching during all growing stages,plastic film-removed after the silking stage increased the annual total N mineralization by 6.5 kg N hm-2 across the 0-40 cm layer, while the difference was insignificant. Compared with the non-mulched treatment, plastic film mulching had no significant effect on soil nitrogen supplying across the 0-40 cm layer during the maize growing season and decreased the risk of mineral N leaching during the fallow season, wheares gravel mulching decreasd soil nitrogen supplying across the 0-40 cm layer during the maize growing season and increased the risk of mineral N leaching during the fallow season.(5)Soil surface mulching and plastic film-removed after the silking stage had significant effects on soil aggregate distribution and stability features after a short term(5 to 6 years). Compared with the non-mulched treatment, gravel and plastic film mulching significantly increased the soil aggregate size distribution and soil aggregate stability in the 0-20 cm layer. Results showed that gravel and plastic film mulching significantly increased the > 0.25 mm macroaggregates(R0.25), mean weight diameter(MWD) and geometric mean diameter(GMD) by dry sieving and wet sieving, meanwhile significantly decreased the fractal dimension(D)、percentage of aggregate disruption(PAD) and unstable index of aggregate(ELT). However, no difference was found between gravel and plastic film mulching in soil aggregate size distribution and soil aggregate stability. Increases in MWD by dry sieving and wet sieving were observed under the plastic film-removed after the silking stage treatment in the 0-20 cm layer, which indicated that plastic film-removed after the silking stage would increase soil aggregate stability.In general, plastic film mulching and plastic film removal at the silking stage could be the best option for sustainably enhancing crop productivity and maintaining soil quality in the semiarid regions of northwest China. |
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