NC_OLD1032: Characterizing active soil organic matter pools controlling soil N availability in maize-based cropping systems (NC218)
Statement of Issues and Justification
The global challenge of meeting ever-increasing demand for food, fiber and energy from maize-based (Zea mays L.) agro-ecosystems requires the efficient use of fertilizer nitrogen (N) resources. Adequate N supply is required for achieving high maize yields. However, improper N fertilizer use threatens environmental quality and human health at both local and global scales through leaching losses to groundwater, hypoxia, surface water degradation, and greenhouse gas emissions. Fertilizer N also accounts for approximately 50% of the fossil energy input to maize production. With rising energy costs, this has become a major impediment to both the profitability and sustainability of maize-based cropping systems.Since the relationship between crop yield and N uptake is tightly conserved, achieving higher yields to meet demand will require greater crop N uptake. This is most efficiently achieved not by simply increasing fertilizer N inputs, rather by also improving N use efficiency and reducing the amount of reactive N that is released to the environment (Cassman et al., 2002.).
The principal determinants of plant-available N supply are the net N release or mineralization of N from soil organic matter and crop residues, contributions from applied inorganic and organic N sources, and losses from the plant-available N pool. Therefore the supply of N from indigenous soil N resources and the synchrony of mineralization in relation to crop demand are key considerations in determining the optimal timing and rate of fertilizer N application and achieving more efficient use of fertilizer N. Since soil contents of C and N are closely related and the soil C:N ratio is relatively stable, cropping practices that enhance primary crop productivity and C input to soil will also increase indigenous soil N and over the long-term result in a reduction in fertilizer N requirement. The magnitude and annual variability of indigenous soil N supply are largely unknown, even though this N pool is of primary importance to designing efficient systems for fertilizer N management.
Indigenous soil N can be affected by diverse factors. For example, the quantity, quality and timing of carbon inputs to soil influence the formation of new soil organic matter and the storage and release of soil C and N. Empirical evidence has long suggested that both fertilizer N use efficiency and maize productivity can be improved through crop rotation. However, new evidence suggests that maize-soybean (Glycine max (L.) Merr.) rotations, the dominant crop rotation practiced in the north-central region, results in declining stocks of both soil C and soil N. (Baker and Griffis, 2005; Verma et al., 2005). Maize is also a rotation crop in many cropping systems for both grain and forage and therefore this research has broader implications as evidenced by the diverse NC-218 membership from different regions.
Improvements recorded in fertilizer N use efficiency in maize- based rotation result from exploitation of indigenous soil N, placing in question the long-term sustainability of maize-soybean rotations. Net mineralization of the indigenous N supply is influenced by cropping sequence, and it also varies greatly over time. These diverse sources of variation complicate the prediction of the indigenous N supply and the development of efficient N fertilizer applications that incorporate measures of indigenous soil N supply. To illustrate this point, no single soil test to predict N availability has been adopted by commercial soil test labs.
The majority of C and N in soil cycles slowly, over time-spans ranging from years to centuries. Only a small fraction of soil associated with decomposer activity and recent organic matter inputs cycles a rate fast enough to affect seasonal N availability. This small fraction of actively cycling N may contain significant quantities of recently deposited organic material including fine roots and fungal hyphae and is often referred to as particulate organic mater or light fraction (Tisdall and Oades 1982). Hence only a portion of indigenous soil C and N is significantly involved in seasonal N cycling, creating the need to study the active fractions of soil C and N in order to predict N mineralization and availability. Specifically we need a more explicit understanding of the nature and dynamics of active soil C and N and how management and cropping system influence the storage and release of both C and N from these pools.
The efforts of the NC-218 committee over the past decade have concentrated on the measurement of indigenous soil N supply through rapid laboratory analyses that are designed to measure the amounts of inorganic N released through chemical treatment of soil C and N or through temporal analysis of soil nitrate. These tests were evaluated and found to be incapable of predicting either the spatial variation in crop uptake of indigenous soil N as influenced by inherent edaphic properties and site productivity or the temporal variation as influenced by cropping system and previous soil and crop management practices. This absence of a broad predictive capability most likely results from the fact that no single laboratory procedure that releases inorganic N from organic N pools can act on all active N pools without simultaneously acting on older, more stabilized N pools that do not contribute to short-term N cycling under field conditions. The committee believes that this statement also applies to other rapid analyses that the committee did not evaluate. Hence the proposed new activities will not further evaluate rapid analyses for soil N availability. Instead a new research direction is proposed for improving the management of indigenous soil N.
Recent work has demonstrated the short-term activity of discrete organic fractions of soil C and N that are extracted intact from soil. The light fraction, which is extracted from the soil based on density, and the mobile humic acid fraction, which is extracted with sodium hydroxide, were shown to be highly involved in seasonal N cycling both in maize-based rotations of Nebraska (Legorreta-Padilla, 2005) and continuous rice fields in California (Bird et al., 2002). The masses and carbon contents of these organic fractions were also shown to be responsive to recent crop management. These two fractions provided complementary insights into the dynamics of short-term N and C cycling, as their constituent materials are bound in the soil by different manners and are at slightly different stages of decomposition. The stability, size, and N contribution/turnover within these pools are influenced by the nature of residue quantity and quality, anthropogenic inputs of N, and the physical environment.
To elucidate the processes that control the availability and crop uptake of indigenous soil N in maize-based rotations, members of the NC-218 committee will conduct a series of measurements on N forms and dynamics in a range of long-term field experiments and farmers fields that encompass a variety of crop rotations and soil types. A core experimental approach will be utilized across this range of experimental environments to measure gross and net N mineralization and the change in soil N and C storage and release from crop residues. Inorganic N and crop N uptake will be measured periodically during the growing seasons, and soil samples will be collected for extraction of the light fraction and mobile humic acid fraction. These fractions will be analyzed for their masses and biochemical natures. One key measurement will be their contents of specific amino acids and amino sugars, using a newly developed method of anion chromatography that identifies nearly 90% of all organic N in soil, a far greater proportion than has been possible with previously available analyses. Experiments on research farms of participating universities will be sampled in order to identify longer-term interactions between crop rotation, tillage and fertilizer treatments that affect cycling of indigenous N. Shorter-term dynamics of indigenous N will be studied in plots of N fertilizer rates that will be installed in farmers fields that have tended to differ from each other in the responsiveness of crop growth to N fertilizer rates. External funding will be sought for application of stable N and C isotopes into some sites for quantifying the rates of specific N processes in the soil, including gross mineralization and gross immobilization. The regional and cross-regional perspectives and the range of environment and expertise provided by the participants will lead to broad applicability of the findings from this research. The use of a common core experimental approach plus the centralized, collaborative arrangements for analytical work and interpretation are advantages of the regional research approach.
Impacts from the successful completion of the proposed work include a quantitative survey of indigenous N supply under a wide range of edaphic and climatic regions as influenced by cropping system and N management. In addition, we will have obtained a more complete understanding of the consequence of cropping systems on soil C sequestration potential and its link to soil N sequestration, indigenous N supply and fertilizer N use efficiency.
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