Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: January 14, 2004
Publication Date: March 2, 2004
Citation: Nichols, K.A., Wright, S.E., Liebig, M.A., Pikul Jr, J.L. 2004. Functional significance of glomalin to soil fertility. p. 219-224. IN: A.J. Schlegel (ed.) Great Plains Soil Fertility Conf. Proc. Vol. 10 Denver, CO. 2-3 March 2004. Kansas State Univ. Meeting Proceedings. Interpretive Summary: Soil aggregation improves soil fertility by: (1) holding nutrients in protected microsites near plant roots, (2) maintaining soil porosity, which provides aeration and water infiltration rates favorable for plant and microbial growth, (3) storing carbon (C) by protecting organic matter from microbial decomposition, and (4) increasing stability against wind and water erosion. Arbuscular mycorrhizal (AM) fungi help form water-stable soil aggregates by physically enmesh soil particles and organic debris. Glomalin is a glycoproteinaceous substance found in abundance (typically, 2 to 15 mg/g and up to >60 mg/g) in a wide range of soil environments acidic, calcareous, grassland and cropland) throughout the world. The polysaccharides and aliphatic amino acids in glomalin and tightly-bound iron on glomalin create a sticky, complex biomolecule that forms a conglomeration with root fragments and organic matter and provides a protective coating for this conglomeration. In this study, the portion of C in organic matter fractions - particulate organic matter (POM), glomalin, humic acid (HA) and humin - and the percentage water-stable aggregates (WSA) were measured in two native grassland soils from eastern Colorado. In both soils, the total portion of C in glomalin was nearly equivalent to the amount of C in the R-POM fraction (ca. 25 to 35%). The distribution of C, or organic matter, within aggregates may indicate the percentage of WSA in the soil as well as how aggregates are formed and stabilized. If aggregates are not water-stable, water will rapidly enter air-filled pores within aggregates which will increase air pressure and cause aggregates to burst. Large aggregates may be disrupted into smaller aggregates which then may be easily carried by wind. Increasing the amount of WSA will help reduce both water and wind erosion by keeping soil particles, organic matter, and mineral nutrients in large conglomerations that cannot be carried easily by water or wind. Maintaining high concentrations of water-stable aggregates also improves rooting environment (e.g. plant health), soil water relationships, and fertility (e.g. mineralization and nutrient availability). Tillage, fallow treatments, and low crop diversity may reduce the percentage of WSA and the presence of AM fungi. This information will be valuable to scientists and farmers throughout the world by encouraging agricultural practices to proliferate AM fungi that produce glomalin while preventing soil degradation.
Technical Abstract: In the U.S., soil is lost to wind and water erosion at a rate of nearly 2 billion tons per yr. The formation of aggregates helps stabilize soil and increase soil fertility. Organic matter concentration is correlated with the percentage of water-stable aggregates (WSA). The hypothesis of this study was that glomalin, a glycoproteinaceous substance produced by arbuscular mycorrhizal fungi, would be a major fraction of organic carbon in WSA. Four organic matter fractions - particulate organic matter (POM), total glomalin, humic acid (HA) and humin - were quantified in 1-2 mm dry-sieved aggregates from two native eastern Colorado grassland soils - Sampson and Haxtun. Each fraction, except for humin which remained in the soil after all extractions, was sequentially extracted from the same aggregate sample. Extraction procedures separated total glomalin into three different fractions: glomalin associated with POM (POM-glomalin), initial glomalin extracted from POM-free soil (glomalin), and recalcitrant glomalin (R-glomalin) extracted from POM-free soil after all other fractions. After extraction of glomalin, the POM fraction was reclassified as residual POM (R-POM). The WSA percentage was measured by wet-sieving. In this study, WSA percentage was 52% for the Sampson soil and 62% in the Haxtun soil. In the Sampson soil, the amount of carbon in the total glomalin and R-POM fractions were almost equivalent and greater than in the humin and humic acid fractions. In the Haxtun soil, the humin fraction accounted for the majority of carbon followed closely by R-POM and total glomalin. The Haxtun soil was a sandy loam with less R-POM and total glomalin, especially R-glomalin, than in the Sampson loam. The R-POM fraction contains many labile polysaccharides to help glue aggregates together as well as roots and fungal hyphae to provide the framework for aggregate formation. Glomalin also contains polysaccharides to glue aggregates together as well as iron to form stable bridges with clay minerals and hydrophobic groups such as aliphatic amino acids. The organo-mineral complexes formed between clay minerals and glomalin or humin and a hydrophobic coating from glomalin help keep aggregates water-stable and protected from water and wind erosion.