Variation of deep nitrate in a typical red soil Critical Zone: Effects of land use and slope position

Abstract

There is evidence that high levels of nitrate are stored in Earth’s Critical Zone between the land surface and impermeable bedrock. While nitrate in the upper layers of the soil (< 1 m) has been extensively studied, less information is available on its variation and the controlling factors at depth (> 1 m). To study controls on nitrate accumulation in a subtropical monsoon area, a total of 728 regolith samples, taken from land surface to fresh bedrock, were collected from 21 drillings in a typical red soil Critical Zone in southern China. We characterized the variation of nitrate in these deep horizons and investigated the effect of land use, slope position and regolith physiochemical properties. The results showed that the amount of nitrate at 240-380 cm depth was more stable than at 100-240 cm or 380-500 cm depth, and accumulated at that depth in the upland and orchard regoliths. Nitrate contents in the deep horizons of upland and orchard regoliths were significantly larger than that in the paddy fields or woodland (p < 0.05). When compared to the regoliths from the upper slope of the upland, those from the middle slope stored significantly more nitrate in the deep regolith layer (p < 0.05). Soil pH was the most important single variable in explaining nitrate variability, followed by particle-size distribution (sand, silt and clay) at the point scale. We found that land-use was the most important control in explaining nitrate variation followed by slope position and finally regolith physiochemical properties. These results suggest that deep regoliths of the uplands and orchards in humid regions are most likely to hold a large reservoir of nitrate, which has routinely been overlooked in the past. Moreover, the dominant physiochemical properties of the regolith in controlling the nitrate levels varied among different layers. The findings could have significant implications for biogeochemical cycling of nitrogen in deep soil layers.