Managing Nitrogen Mineralization Potential and Biologically Active Organic Matter Fractions in Agricultural Soils

Willson, T., E.A. Paul, and R.R. Harwood

Presented at the All Scientist Meeting (1998-07-21 to 1998-07-22 )

As in most terrestrial ecosystems, the productivity of row-crop agriculture is often limited by the availability of inorganic N. Nitrogen deficiency can be overcome by the use of inorganic N fertilizers or by the management of biological resources such as plant residues and organic fertilizers. An oversupply of inorganic N from either source can lead to economic losses and environmental contamination through increased leaching and denitrification. As farmers increase their use of biological resources in search of more sustainable production strategies, it will become increasingly important to determine how their management decisions effect the supply of inorganic N and to be able to adjust fertilizer recommendations accordingly.My dissertation research focuses on the effects of agricultural management and early plant succession on the storage and release of organic C and N in KBS soils. I compared the seasonal changes in N mineralization potential (NMP) with seasonal changes in C and N content of microbial biomass (MB) and macrooganic matter (MM) across a variety of corn-based agricultural rotations and successional treatments the LTER main site and the nearby Living Field Laboratory (LFL). I measured NMP based on the accumulation of inorganic N in soils during long term (150 or 310 day) laboratory incubations at 25° C. Microbial biomass was measured using the chloroform fumigation, incubation method (CFIM) and MM was defined as the C and N content in the 53-250 m m and 250-100m m size classes of dispersed soil separates.Figure 1 shows the relationship between the accumulation of inorganic N in laboratory incubations and the MM C content of soils from LTER treatments 1 (Conventional), 7 (Historically Tilled), 8 (Never Tilled) and the tilled microplots in treatment 7 (Annually Tilled) across 5 sample dates in 1995 and 1996. The untilled successional treatments (7 and 8) had greater NMP than either the tilled successional treatments or any of the agronomic treatments. Among the agronomic treatments, those that received organic fertilizer in the form of compost and those that incorporating leguminous cover crops as green manure tended to have greater NMP than treatments receiving inorganic fertilizers and no cover crops. Nitrogen mineralization potential generally decreased from April to early fall (September or October) in all treatments, although it tended to recover in November.The strong correlation between MNP and MM shown in Fig, 1 was consistent across all treatments and dates in this study, although MM tends to reach maximum and minimum levels somewhat later than NMP. As a result, the relationship between NMP and MM C tends to change throughout the season (Fig. 2) with greater NMP per unit MM C in April and November than in October. A similar pattern was found in 1995-96. Our interpretation is that NMP reflects both mineralizable N in MM and the mineralizable N in recently deposited plant residues. The former is relatively constant over the season while the latter reaches a minimum in September-October.Microbial biomass was not correlated with N mineralization, although it was correlated with 10 day C mineralization. There was a dramatic decline in MB C in all treatments in June, 1994, apparently the result of a prolonged drought that spring. Microbial biomass tended to peak in the warm summer months (July-September), which also suggests that environmental conditions rather than substrate availability controls seasonal changes in microbial biomass at this site.These results are being submitted for publication as references 1 and 2 below. A third paper based on this study (reference 3) generates a simplified N budget for the LFL treatments and compares the NMP, MM C and N, and MB C and N measurements at each sample date to estimated field mineralization, yield, crop uptake and loss in the current and subsequent growing season in an attempt to determine which of these measurements was is the best measurement of N availability, crop performance, and N loss.Additional projects not related to my dissertation include the below ground productivity project (BGP) in which we are using 13C natural abundance to follow corn and alfalfa C into soil fractions and mineralization pools and a fertilization experiment in which I use 15N labeled fertilizer and 13C natural abundance to follow the fate of sidedress fertilizer N and corn C over two years of corn production. These papers should be completed this fall.T.C. Willson, O. Schabenberger, E.A. Paul, R.R. Harwood. (undergoing final corrections) Seasonal changes in nitrogen mineralization potential in agricultural soils: ANOVA and non-linear regression analysis of management effects. (will be submitted to Soil Science Society of America Journal)T.C. Willson, E.A. Paul, R.R. Harwood. (undergoing final corrections) Management of biologically active soil organic matter fractions for sustainable crop production. (will be submitted to Applied Soil Ecology)T.C. Willson, M.E. Jones, E.A. Paul, R.R. Harwood. (draft) Biological indicators of crop performance, nitrogen mineralization, and leaching loss in integrated cropping systems. (Journal yet to be determined)Return to Contents

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