Nitrogen Conservation in High-Input Low Diversity and Low-Input High Diversity Agroecosystems.

Agroecosystem intensification and diversification are two divergent approaches to meet the growing demand for food. Intensification relies on large exogenous inputs of energy, herbicides, and fertilizers to maximize grain production. Diversification relies on rotation of several crops and manure recycling to decrease exogenous inputs and increase biological nitrogen fixation. We propose a new conceptual model for nitrogen cycling and retention that shifts the focus of nitrogen management from inorganic nitrogen pool size to inorganic nitrogen production rate. We hypothesize that diverse systems have smaller inorganic nitrogen pool sizes but larger inorganic nitrogen production rates that maximize inter-annual grain yield and agroecosystem resilience to rapid nitrogen losses. Trends in preliminary data from the Marsden Farm cropping systems diversity experiment in Iowa, USA suggest diversified systems conserve nitrogen by maintaining smaller nitrate pool sizes but higher nitrogen mineralization rates. In contrast to diversified systems, many intensified systems lack high organic matter manure additions. Due to lower organic matter, these systems rely on relatively large inputs of nitrogen fertilizer to maximize inorganic nitrogen pool size, crop yield, and crop residue organic matter input to soil. In these systems, optimization of nitrogen fertilizer rate is required to conserve soil nitrogen due to the effect of nitrogen fertilizer on crop residue production: insufficient nitrogen applications decrease crop residue production and ultimately soil nitrogen pools. Using four long-term continuous corn and corn-soybean rotation systems in Iowa USA, we report negative soil nitrogen budgets when nitrogen fertilizer rates are insufficient to optimize yield. When managed at agronomic optimum, continuous corn maintains positive soil nitrogen balances. However, in three of four sites, soil nitrogen balances were negative in corn-soybean rotations despite optimal nitrogen fertilizer rate and maximum crop residue production. In corn-soybean rotations, crop residue production was too little to maintain positive soil nitrogen balances despite optimum management.