The N-cycle as determined by intensive agriculture – examples from central Europe and China

Abstract

Using a scientific assessment concept of sustainability in crop-production based on the entropy production minimization principle of thermodynamics, formation and non-use of soluble and volatile (by-)products of the nutrient cycles within the system are interpreted as indicators or measures of the low efficiency/sustainability of recent forms of intensive agriculture. The simultaneous high energy input in modern crop production systems further shows the difference between these and quasi-stationary natural systems with maximum bioproduction having minimum energy dissipation and entropy production. Using balance sheets and dynamic approaches, the practical implications regarding the nitrogen cycle in central Europe (FR Germany) and China are exemplified and discussed. The average N balance of arable systems in Germany shows surplus N amounts of 110–130 kg N ha^-1 yr^-1. A high N immobilization in accordance with deepened top soil layers has governed N balances in Germany since about 1960. In China Nbalance surpluses in intensive agricultural (double-cropping) systems on the southern edge of the Loess Plateau now reach 125–230 kg N ha^-1 yr^-1. In field experiments, mineral N contents in the profiles (0–1.2 m depth) were 72–342 and 78–108 kg ha^-1 at harvest of summer maize and winter wheat, respectively. In the Taihu region in eastern China, surpluses in the N balance (rice-wheat double cropping) amount to 217–335 kg N ha^-1 yr^-1. N_min contents in the 0–0.9 m profiles of between 50 and 100 kg N ha^-1 were frequently found after winter wheat harvest. In two separate investigations of ground and well water samples in China, nitrate contents exceeded the critical WHO value for drinking water in 38–50% of the locations investigated.