Do irrigated and rainfed agrosystems change aboveground and root carbon and nitrogen stocks in a semiarid environment?

Abstract

Determining shoot and root biomass and carbon (C) and nitrogen (N) stocks in different ecosystems is crucial to propose adaptative measures to reduce greenhouse gas emission. Data on systems of semiarid areas are scarce, especially irrigated ones. We compared biomasses of four irrigated (bean, maize, grapevine, and mango) and four rainfed livestock systems (buffel grass, prickly pear, gliricidia and leucaena) with those of a preserved and an open tropical dry forest (Caatinga) used as pasture. Caatinga biomass was estimated by allometry and biomasses of the other systems by the destructive method. Root biomasses were determined in layers down to 100 cm. Mango had the highest aboveground and root biomasses: 34.2 and 91.3 Mg ha⁻¹. Preserved Caatinga was the second in aboveground biomass: 25.1 Mg ha⁻¹, compared 9–14.1 Mg ha⁻¹ of the open Caatinga and permanent crops and 3.0–11.8 Mg ha⁻¹ of the grass and annual crops. Preserved Caatinga, gliricidia and buffel grass formed the second group in root biomass (23.7–25.1 Mg ha⁻¹), while all other systems had less than 13.5 Mg ha⁻¹. Preserved and open Caatinga, annual crops and prickly pear had root:shoot ratios below 1, while the other systems had ratios >1, especially mango (2.7). Most systems had roots concentrated in the 0–15 cm layer, but the permanent crops, except prickly pear, concentrated about half of their roots in deeper layers. C (34.2–46.6%) and N (0.6–3.4%) concentrations varied less than biomasses. Root biomass was the plant factor mostly correlated with soil C and N stocks. Therefore, irrigated crops can accumulate more biomass than the native forest, but substitution by mango led to a severe depletion in soil C and N stocks, and all other systems also lost soil C and N. Management strategies such as green manure and crop diversification should be adopted to maintain plant and soil C and N stocks similar to those of the original vegetation.