Nitrogen economy in relay intercropping systems of wheat and cotton

Abstract

Relay intercropping of wheat and cotton is practiced on a large scale in China. Winter wheat is thereby grown as a food crop from November to June and cotton as a cash crop from April to October. The crops overlap in time, growing as an intercrop, from April till June. High levels of nitrogen are applied. In this study, we analyzed the N-economy of the monocultures of cotton and wheat, and of four relay intercropping systems, differing in number of rows per strip of cotton or wheat. Field experiments were carried out from 2001/02 to 2003/04 in the Yellow River region in China. We quantified the nitrogen uptake and nitrogen use efficiency of wheat and cotton in relay intercropping systems to test if intercrops are more resource use efficient in comparison to monocrops. Nitrogen (N) yields of wheat per unit area in the four intercropping systems were lower than in the monocrop, which ranged from 203 to 288 kg ha−1. The total N-uptake per unit biomass was similar between wheat in mono- and intercrops. On average, the N-yield of cotton per unit area was lower in intercrops than in monocrops, which ranged from 110 to 127 kg ha−1, but the total N-uptake per unit biomass was higher in intercropped cotton, as dry matter production was reduced to a greater extent by intercropping than N-uptake. The N-uptake of cotton was diminished during the intercropping phase, but recovered partially during later growth stages. The physiological nitrogen use efficiency (IE) of wheat was not much affected by intercropping, but it was reduced in cotton, due to delayed flowering and less reproductive growth. Total N-efficiency of the system was assessed by comparing the relative nitrogen yield total (RNT), i.e. the sum of the ratio’s of total N-uptake by a component crop in the intercrop relative to the N-uptake in the monocrop, to the relative yield total. RNT ranged from 1.4 to 1.7, while the relative yield total (RYT) ranged from 1.3 to 1.4, indicating that intercrops used more nitrogen per unit production than monocrops. An analysis of the crop nitrogen balance showed that the nitrogen surplus of sole crops amounted to 220 kg ha−1 for wheat and 140 kg ha−1 for cotton, while in the intercropping systems, the annual N surplus exceeded 400 kg ha−1. Conventional N-management in intercrops thus results in high N-surpluses that pose an environmental risk. The N management could be improved by means of a demand-based rate and timing of N applications.