ABSTRACT
Agroecosystems are endowed with large quantity of mineral nutrients and organic matter that are held in bed rock, soil phase, crop phase, atmosphere, water bodies and aquifers. Nutrients are translocated/retranslocated from each aspect of agroecosystem to others. For example, nutrients are absorbed/recovered from substrate (soil) to crop phase. Similarly, nutrients are transferred from crop to soil through recycling of residues, then from soil to atmosphere via emission and from soil surface to ground water through percolation. Nutrients are also shifted away from an agroecosystem through harvest and transport to long distances from original fields. In the present context, our focus is on the net nutrients recovered from soil to crop phase and its accumulation into different plant parts. The consequences of nutrient recovery from soil on crop growth, forage and grain formation is also important. Nature abounds with variations in terms of crops/mixtures that dominate an agroecosystem, rates of nutrients recovered and total quantity of nutrients held in the crop phase at different stages. Several factors related mostly to geographic location, topography, agroclimate, soils, precipitation and water resources, crop species, nutrient supply rates, and yield goals affect nutrient absorption/recovery rates in a given agroecosystem. Let us discuss a few agroecosystems with regard to above aspects. Geographically, there are regions on earth that support intensive, high input, high yielding agroecosystems. For example, Corn Belt in USA, Wheat cropping zones in Western European plains, Wheat production zones in Northeast China, Rice in Cauvery delta zone of South India, Rice belts in Far-eastern regions etc. High productivity automatically means proportionately massive absorption of soil nutrients into above-ground portions and grains/tubers/fruits. The nutrient absorption rates per time and per unit area are high. Plant tissues accumulate relatively greater amounts of nutrients. To quote a few examples, Wheat grown in Northern Great Plains of United States of America is said to recover at least 81-85 kg N/ha to yield 3.5-4.2 t grains/ha [1]. Over a large expanse of >100,000 ha in Western Europe, a wheat crop that yields 7.5 t grain/ha removes 190 kg N/ha. About 140 kg N/hacould be traced in the grain and the rest 50 kg N/ha in stover that can be either recycled or used to feed animals [2]. In the Australian wheat belt, N recovery rate at different stages of the crop on Zadock’s scale has been documented. Usually, about 60-80 kg N/ha is recovered from soil phase. A crop that produces 2.2 t/ha partitions 25-40 kg N/ha into grains. In Argentinean Pampas, a wheat crop that forms 2.27 t grain/ha recovers 45-50 kg/ ha. Wheat crop may actually absorb 14-20 kg N/ha to form 1.0 t grain plus forage depending on the harvest index. The exact amount of N recovered, of course depends on the agroecosystem, its soil fertility status, crop genotype and yield goals. Nutrient recovery by a crop may vary depending on general agroclimate, season and crop rotations followed. Whatever is the geographic location, soil fertility status, water supply level, agronomic procedures, or even crop genotype and yield goals set, we ought to realize that physiological genetics of a crop species stipulates quantity of nutrients to be removed to produce a unit, say one ton of grains and foliage.
