Re-thinking soil nitrogen availability to crops in the context of soil organic carbon

Re-thinking soil nitrogen availability to crops in the context of soil organic carbon

Abstract

The link between carbon and nitrogen in soil organic matter has long been recognized but has been largely ignored in determining the supply of nitrogen from soil for crop production. We propose that nitrogen mineralization can only be understood as a by-product of soil organic carbon change and that progress in improving nitrogen use efficiency by crops must explicitly include consideration of soil organic carbon. This suggests some immediate avenues for improved fertilizer nitrogen management as well as future research activities. Recent advances in understanding the various fractions of soil organic matter and their role in carbon cycling have great potential for improving our understanding of nitrogen cycling in the soil as well, and the potential for nitrogen release that could be utilized by crops.

One. Introduction

Providing an adequate supply of nutrients, including nitrogen, is key to maintaining economic yields of grain, forage, or fiber crops. Supplemental nitrogen fertilizer is an integral component of crop production, particularly for nonleguminous crops, with estimates that the world would require up to four times the crop area to generate current production without the use of nitrogen fertilizer. It is also important to note that nitrogen availability is an important regulator of how much carbon is fixed by the crop through photosynthesis and therefore an important determinant of the potential to convert carbon dioxide from the atmosphere into plant biomass, part of which will contribute to soil organic matter. The use of nitrogen fertilizer, however, comes with both economic costs and environmental risks so there is a critical need to maximize the efficiency of nitrogen use. One way to improve nitrogen efficiency is to account for the supply of mineral nitrogen from the organic nitrogen pool in the soil (nitrogen mineralization).

The apparent goal of accounting for soil organic nitrogen mineralization has been to identify the opportunity to replace supplemental nitrogen fertilizer with nitrogen being supplied from the soil. Significant effort has been expended to account for the contribution of soil organic nitrogen for corn, potatoes, vegetables, and wheat, but the accuracy of prediction tools has been relatively low. As a result, nitrogen mineralization from soil organic matter is often not quantitatively considered in determining supplemental nitrogen fertilizer application rates, although some jurisdictions include an empirical factor for soil texture, which is conjectured to be related to the soil organic carbon content of the soil.

A key missing piece to this puzzle has been that these studies have been carried out without considering the inextricable link between nitrogen mineralization and soil organic carbon. A recent paper begins to address this issue but the discussion is limited to cycling occurring during the time when plants are actively growing. The broader context of nitrogen and carbon stoichiometry over multiple years is ignored, as there is no accounting for the nitrogen removed in the harvested portion of the crop, carbon and nitrogen returned in the nonharvested portion of the crop or tied up in the microbial biomass and the implications of this balance on carbon and nitrogen cycling. The authors do suggest a possible mechanism for tighter cycling of nitrogen mineralized from soil organic matter back into organic forms when soil mineral nitrogen concentrations are limited, with an associated reduction of nitrate leaching, but the quantities involved appear to be much less than crop removal.

If the nitrogen requirements of crops are to be met fully or partially from the soil, there are a limited number of sources. In some cases, there will be residual mineral nitrogen in the soil, as is commonly the case in prairie environments and occasionally in humid environments. The larger and more consistent source, however, is the mineralization of nitrogen from soil organic matter, which is then taken up by crops and either recycled back to the soil with crop residues at the end of the season or removed in the harvested portion of the crop. The decomposition of soil organic matter results in nitrogen mineralization at a rate determined by the carbon to nitrogen ratio of the soil organic matter and moderated by soil temperature and moisture contents which in turn influence microbial activity.

It is important to acknowledge the seasonality of nitrogen mineralization and immobilization where in most temperate climates there is net mineralization in the early part of the growing season driven by the microbial decomposition of dynamic pools of narrow carbon to nitrogen ratio less than ten soil organic matter, followed by crop uptake of the mineralized nitrogen during the summer and finally the net immobilization due to the senescence and the return of the typically wide carbon to nitrogen ratio greater than twenty crop residues (roots and shoots) in the fall from nonleguminous plants. It is the wide carbon to nitrogen ratio of crop residues that causes net immobilization of at least some of inorganic nitrogen remaining in the soil in the fall because of nitrogen demands of microbial biosynthesis. We now understand that soil organic matter is not a single static pool but is composed of numerous pools ranging from stabilized soil organic matter, which constitutes the largest pool, and several more varying proportions of dynamic or labile pools that remain biologically active. Typically, net nitrogen mineralization occurring in the early growing season averages sixty to one hundred thirty kilograms of nitrogen per hectare for most growing regions in Canada but is influenced by soil type, climate, soil management, and cropping system. In most temperate cropping systems roughly fifty percent of plant nitrogen taken up during the growing season is derived from mineralized soil organic matter with the remainder provided by supplemental nitrogen sources (fertilizer, manure, legumes) and to a lesser degree from atmospheric deposition, and asymbiotic nitrogen fixation by soil microbes.

In the absence of a supplemental source of nitrogen, the removal of nitrogen from the field in grain or forage represents a net export of nitrogen. This is a nonequilibrium situation where a portion of the soil organic nitrogen has been mineralized but is not returned to the field at the end of the growing season. This will inevitably be accompanied by either an outright loss of soil organic carbon, or a shift from stable mineral-associated organic matter or microbial necromass to less stable particulate organic matter, which can have a wider and more variable carbon to nitrogen ratio. Previous work on nitrogen availability from soil has focused on mineral nitrogen without accounting for any changes in long-term stocks of soil organic carbon, or on changes in soil organic matter without considering potential impacts on future nitrogen availability. To build or maintain soil organic carbon, it is important that sufficient root exudate and crop residue carbon is returned to the soil to offset the decomposition of soil organic matter. Failing to do so is counter to current efforts to maintain or improve soil health, of which soil organic carbon is a key part.

The release of mineral nitrogen into the soil is a by-product from the breakdown of organic matter, so the availability of nitrogen for crop uptake from the soil can only be understood as an end product of soil organic matter decomposition. Where nitrogen is available from the soil to support significant crop removal, this must have come through the decomposition of soil organic matter but this nitrogen availability is a symptom of the conditions that led to this decomposition from soil including:

· Changes in capacity of soil to retain carbon due to tillage or residue management.

· Weather conditions conducive to soil organic matter decomposition.

· Past accumulation of soil organic matter through additions of organic materials (manure, compost) or deposition of eroded topsoil in the toe slope areas.

We propose that it is the decomposition of soil organic matter that drives the supply of nitrogen from the soil, so that improved nitrogen management for crops can only be achieved in the context of soil organic carbon cycling in the soil. With nitrogen mineralization a consequence of soil organic matter breakdown, we must endeavor to manage the nitrogen that is mineralized as efficiently as possible to maximize utilization by plants and to avoid loss to the environment and that sufficient organic residues are returned to the soil to maintain soil organic matter. Nitrogen that is held in organic compounds, either in plant tissue or soil organic matter, is at much lower risk of environmental losses than mineral forms of reactive nitrogen. In a way, this is parallel to the dilemma that Janzen posed for soil carbon, of determining the proper balance between storage, maintaining the stable pool, and utilization, making best use of the nitrogen flowing through dynamic pools. Dharmakeerthi and Kay did attempt to account for the soil organic carbon content of the soil, but as a static property which varied across the landscape and not in the context of soil organic carbon change.

Nitrogen management for annual crop production will, therefore, operate at two different temporal scales. The first is seasonal, where the rate of release of nitrogen from soil organic matter and the existing store of mineral nitrogen are considered in the context of the uptake of nitrogen by the crop, with a goal of minimized nitrogen loss. The goal in this phase is to utilize the nitrogen that is released during soil organic matter breakdown as efficiently as possible so as to minimize the requirements for added mineral fertilizers. This will also reduce the risk of excess mineral nitrogen in the soil at the end of the growing season, and so helps to minimize the nitrogen footprint of crop production. The second phase is the management of carbon rich above and below ground crop residues such that the formation of stable soil organic matter can occur. Within a growing season, nitrogen exports from a cropping system will be the sum of nitrogen in the harvested portion of the crop and losses from the system through volatilization, leaching, and denitrification. The nitrogen for these export pathways will come from nitrogen additions (fertilizer, manure, legumes) plus the change in soil organic nitrogen over that season, and any exceedance over what is required for crop growth will shift the balance toward nitrogen loss pathways, either during the growing season or post-harvest. This annual balance will constrain the maximum efficiency that can be achieved within the seasonal nitrogen cycles.