Relevance of the soil microbial biomass (including rhizobia and mycorrhizae) to organic-matter turnover, soil fertility and physical structure is outlined, and methods to measure the biomass are discussed. It is suggested that measurements of the soil microbial biomass can be of assistance in soil management decisions.

A no-tillage (NT) chronosequence that had been continuously cropped to maize (Zea mays L.) for 0, 1, 2, 4, 7, 9, 15, or 20 yr on a Westmoreland silt loam (fine-loamy, mixed, mesic Ultic Hapludalf) was examined for differences in microbial biomass, and soil organic C, N, P, and S. In the plowzone of the NT sites, biomass-C, total C (TC), soluble organic C (SOC), total Kjeldahl N (TKN), organic P (OP), and organic S (OS) levels were generally greater in the soil surface (0 to 7.5 cm) layers than in the 7.5- to 15-cm layers. In contrast, biomass-C under conventional tillage (CT or 0-yr NT site) in the soil surface layer was ∼50% of that in the 7.5- to 15-cm layer, whereas levels of the organic components were nearly identical. Biomass-C and organic component levels in the soil surface layers under NT were from 27 to 83% greater than those under CT. Opposite tillage method effects on these properties were usually found for deeper soil layers. Soil organic components, but not biomass-C, were significantly (p ≤ 0.05) related to years under NT in the soil surface layer. Only biomass-C was significantly (p ≤ 0.10) related to years under NT in deeper soil layers. When just “typical (i.e., nonmanured, moderate N-rate)” sites were included in the regression models, only biomass-C and SOC reservoir contents (total to 45 cm) varied significantly (p ≤ 0.10) with years under NT. Soil biomass-C reached a maximum (786 kg·ha furrow-slice⁻¹) in the soil surface layer after only 1 yr under NT, approaching a level nearly equivalent to that under an improved pasture, then equilibrated in about 10 yr to a level approximately 30% greater than that under CT. These observations suggest that continual NT induces a predictable dynamic in soil biomass-C, but not soil organic components, that is generally insensitive to a range of management differences. As a consequence, management practices designed to improve nutrient use-efficiency, especially by controlling microbial mineralization/immobilization activity, should not only take into consideration tillage method but years under the tillage method as well. Contribution from the USDA-ARS, North Atlantic Area.

The incorporation of P derived from fertilizer and plant residues into the soil microbial biomass was studied under field conditions by using isotopic labelling. The 33P-labelled medic residues (Medicago truncatula cv. Paraggio) and 32P-labelled fertilizer were added to a solonized brown soil (Calcixerolic xerochrept) before sowing of a wheat crop (Triticum aestivum cv. Warigal). Amounts of 31P, 32P and 33P in the microbial biomass were determined at 0, 7, 18, 32, 36, 61, 81 and 95 days after sowing. Amounts of 31P in the microbial biomass were closely related to gravimetric soil water content. Due to banding of the fertilizer at sowing, little of the 32P was recovered in the microbial biomass. Of the 33P applied in the medic residues, 22-28 percent was recovered in the microbial biomass. Most of the P taken up by the microbial biomass was derived from native soil P.

A no-tillage (NT) chronosequence that had been continuously cropped to maize (Zea mays L.) for 0, 1, 2, 4, 7, 9, 15, or 20 yr on a Westmorelandsilt loam (fine-loamy, mixed, mesic Ultic Hapludalf was examined for differences in microbial biomass, and soil organic C, N, P, and S. In the plowzone of the NT sites, biomass-C, total C (TC), soluble organic C (SOC), total Kjeldahl N (TKN organic P (OP), and organic S (OS) levels were generally greater in the soil surface (0 to 7.5 cm) layers than in the 7.5- to 15-cm layers. In contrast, biomass-C under conventional tillage (CT or 0-yr NT site) in the soil surface layer was approximately 50% of that in the 7.5 to 15-cm layer, whereas levelsof the organic components were nearly identical. Biomass-C and organic component levels in the soil surface layers under NT were from 27 to 83% greater than those under CT. Opposite tillage method effects on these properties wereusually found for deeper soil layers. Soil organic components, but not biomass-C, were significantly (p less than or equal to 0.05) related to years under NT in the soil surface layer. Only biomass-C was significantly (p less than or equal to 0.10) related to years under NT in deeper soil layers. When just "typical(i.e., nonmanured, moderate N-rate)" sites were included in the regression models,only biomass-C and SOC reservoir contents (total to 45 cm) varied significantly (p less than or equal to 0.10) with years under NT. Soil biomass-C reached a maximum (786 kg.ha furrow- slice-1) in the soil surface layer after only 1 yr under NT, approaching a level nearly equivalent to that under an improved pasture, then equilibrated in about 10 yr to a level approximately 30% greater than that under CT. These observations suggest that continual NT induces a predictable dynamic in soil biomass-C, but not soil organic components, that is generally insensitive to a range of management differences. As a consequence, management practices designed to improve nutrient use-efficiency, especially by controlling microbial mineralization/immobilization activity, should not only take into consideration tillage method but years under the tillage method as well.

Organic matter contains reactive carboxyl, phenolic and amino groups which are capable of bonding hydrogen ions. Such hydrogen ion saturated groups behave as a weak acid and the covalently bound hydrogen ions will dissociate when the dissociation constant is reached. A pot experiment was conducted in glass house at the University of Queensland, St. Lucia, Queensland, Australia from August through September of 1985 to determine the effect of ipil-ipil biomass incorporation into an Australian Yellow podzolic soil on soil pH and aluminum. To simulate what was happening to soil pH and aluminum in a natural environment when ipil-ipil was turned over as a green manure, ipil-ipil biomass was incorporated at rates of 0, 10, 20, 30, and 40 t/ha. Soil pH and aluminum were monitored at a predetermined time and it was found that soil pH was increased significantly (P=0.01) by the addition of biomass but the effect was only temporary. The biomass also played a role in controlling the levels of both soluble and exchangeable monomeric and polymeric aluminum. Higher levels of biomass incorporation have a better control of the aluminum in the soil solution but emphasis should be given to monomeric aluminum because recent findings have established that the aluminum phytotoxicity was due to the monomeric aluminum species.

Seven main community types were found in this study area; pure Imperata cylindrica Beauv., Imperata cylindrica Beauv. mixed with Eupatorium odoratum Linn., Imperata cylindrica Beauv. mixed with Saccharum fusum Roxb., Imperata cylindrica Beauv. mixed with Eupatorium odoratum Linn. and Saccharum fusum Roxb, Imperata cylindrica Beauv. mixed with Hedyotis capitellata Wall., Imperata cylindrica Beauv. mixed with Eriocaulon henryanum Ruhle, and Imperata cylindrica Beauv. mixed with Cratoxylum formosum (Jack) Dyer and Holarrhena antidysenterica Wall. For the chemical properties of soil at 0-5 cm. depth, biomass was highly correlated with the amount of phosphorus in negative mean while at 10-15 cm. of soil depth, biomass was highly positive correlated with pH and negative with potassium. At 20-25 cm. of soil depth, the biomass was positively correlated with pH. At 45-50 cm. of soil depth, the biomass was highly positive correlated with pH and negative with phosphorus.

Aquatic weed biomass was amended to field plots of a Freehold sandy loam (Typic Hapludults) either as a mulch or incorporated into the surface soil with the objective of determining the potential for using soil as a repository for excess weed biomass and for using this biomass to augment soil organic matter levels and associated biological processes. Potential and actual dehydrogenase activities, soil organic C, total Kjeldahl N, and the C/N ratio of the soil were measured. Tomato plants (Lycopersicon esculentum Mill.) were grown in the plots. Amendment of the soil with aquatic weeds annually for up to 3 yr resulted in two- to threefold increases in actual and potential dehydrogenase and total Kjeldahl N. No consistent effect of inclusion of nitrogenous fertilizer with the aquatic weed amendments was detected. Analysis of the data with multiple linear regression techniques indicated that total Kjeldahl N and soil moisture were major controllers of both potential and actual dehydrogenase activities. This study suggests that aquatic weeds, whether added as a mulch or incorporated into the surface of the soil profile, have little effect on soil C, at least in the short run, but have a positive impact on the ecosystem through augmentation of the soil organic N pools and increased microbial activity. New Jersey Agric. Exp. Stn. pub. D-15288-1-86.