Interpreting soil organic matter characteristics in organic farming systems

Summary

Soil organic matter (SOM) fraction characteristics need to be developed as indicies that can be related to specific soil functions. The general effects of organic management on prominsing indicies of SOM status (POM and a rapid measure of base hydrolyzable N) were determined by comparing the characteristics of organic and conventionally managed soils obtained from nine long-term trials in North America. In addition, a more detailed study was conducted of POM and of hydrolyzable N fractions (including amino-acids, amino sugars) at a site where use of organic practices failed to increase SOM. In the multi-site comparrison, legume- and manure-based organic systems performed equally well in their ability to increase the quantity of SOM as well as enrich the proportion of particulate organic matter, where POM was assessed using a variety of methods. The quantity of C and N in the coarse fraction, loose light fraction, and aggregate occluded fraction were similar in soils from legume and manure-based systems. The amount of POM-N recovered using a variety of methods was equal to the amount of N recovered by base hydrolysis and was more that twice that required to support a full crop of maize. Base hydrolyzable N was no more sensitive to management than was total N. We were able to differentiate between the manure- and legume based organic systems by assessing the quality of the aggregate occluded POM fraction (< 2.0 g cm-1); this fraction was most humified and contained a greater amount of plant available N in the manured soils. In a related study of an aberant organic site, where SOM levels remained low and maize yields lagged in the legume based system, the only SOM attribute that differed among the three systems was amino acid N contents. Contents were least in the conventional and greatest in the manure based organic system. Whole soil and POM C/N ratios were quite low and were accompanied by extremely high amino sugar levels. Fraction characteristics suggest an imbalance in soil C and N reserves and that disease, rather than N insufficiency, may be limiting yield. This, and possibly C priming by surplus N, may constrain organic matter accumulation. Through using multiple measure of labile SOM, and considering C and N together, we begin to use SOM diagnostically. Disproportional responses in labile fractions can identify resource limitations or surpluses within a system.