Response of soil respiration to hydrothermal effects of gravel-sand mulch in arid regions of the Loess Plateau, China

Gravel-sand mulching is a traditional no-tillage technique used for more than 300 years on the Loess Plateau. Applying gravel-sand mulch (mixture of gravel and sand) has an impact on the soil environment, soil microorganisms, and crop yield. However, we know surprisingly little of how respiration dynamics at the soil surface is affected by gravel-sand mulching. To study the response of soil respiration (Rs) dynamics to different in gravel-sand mulch thickness and its driving factors, we recorded Rs under a gradient of gravel-sand mulch thickness consisting of six layers (0 cm [CK], 3 cm, 5 cm, 7 cm, 9 cm, 11 cm), in situ, during the growing season by using the LI-6400-09 soil respiration measurement system on the Loess Plateau in northwest China. The trends of all six treatments in diurnal and monthly variation of soil respiration followed a single-peaked curve; the diurnal dynamics trends were consistent with soil temperature changes. With gravel-sand mulch applied, the daily peak of soil respiration decreased, and the peak in Rs under the gravel-sand mulch treatments lagged that of bare land by 2 h. Monthly changes in soil respiration were consistent with soil temperature and precipitation changes, with peaks concentrated in July and August. Compared with bare land (CK), the monthly average soil respiration is lowest at 0.67 mu mol center dot m-2 center dot s-1 under a 11-cm thick mulch layer, and reduced by 26.63-61.03% by all mulch applications. Gravel-sand mulch reduced the temperature sensitivity of soil respiration, while soil temperature and soil water content were the main factors determining soil respiration. The thickness of gravel-sand has a significant, negative exponential function relationship with the changes in monthly soil respiration. Further, compared with bare land, the presence of gravel-sand mulch reduced the positive impact of precipitation on Rs. Soil temperature (T), gravel-sand mulch thickness (M), and soil pH were all significant predictors of soil respiration (Rs) (multiple regression model: Rs = 8.561 + 0.077 T - 0.056 M - 0.989 pH), together accounting for 73% of its variation. Hence, by considering all these three factors simultaneously we can better explain the changes to soil respiration in response to gravel-sand mulch. Overall, our results show that as the thickness of the gravel-sand layer increases, soil respiration decreases.