Determining nitrogen and carbon footprints to reveal regional gross primary productivity and differentiation characteristics in karst and non-karst watersheds, China

Abstract

This study revealed the difference of regional gross primary productivity (GPP) in two different geomorphic provinces. We used δ13C, δ15N, and δ18O to trace carbon (C) and nitrogen (N) footprints in two different watersheds in China and analyzed differences in regional economic development. The total dissolved total nitrogen (DTN) and dissolved carbon (DC) deposition fluxes were 16.93 kg N·hm−2·yr−1 and 27.67 kg C·hm−2·yr−1 in the Chenqi watershed (karst) and 43.88 kg N·hm−2·yr−1 and 49.15 kgC·hm−2·yr−1 in the Xiangxi watershed (non-karst), respectively. Rainwater was more enriched in δ13C in the Xiangxi watershed (−14.4‰±1.89‰) than in the Chenqi watershed (−8.54 ± 3.87‰). In addition, DTN efflux was 244.22 and 941.61 kg N·hm−2·yr−1 and efflux of DC was 4157.86 and 9129.22 kg C·hm−2·yr−1 in surface water and groundwater in Chenqi watershed. And DTN efflux was 413.68 kg N·hm−2·yr−1 and DC was 2846.05 kg C·hm−2·yr−1 in Xiangxi watershed. Nitrate nitrogen (NO3−-N) and dissolved inorganic carbon (DIC) were the main forms of DTN and DC, respectively in both watersheds (both >84%). Carbonate reaction and groundwater were the main sources in the Chenqi watershed (contribution rates >30%), while atmospheric deposition and soil water were the main sources in the Xiangxi watershed (contribution rates are >22% and >28%, respectively). Using C and N footprints, we speculated that the direct causes of economic disparity in the two watersheds were nutrients lost with soil erosion which limited vegetation growth. Therefore, it is necessary to plant the suitable vegetation rationally. This study also provides a scientific basis for economic development and ecological environment coordinated sustainable development.