Distribution of soil microbial biomass and potentially mineralizable nitrogen (PMN) in long-term tillage comparisons at seven sites in the United States varied with tillage management and depth in soil. Microbial biomass and PMN levels of no-tillage soils averaged 54% and 37% higher, respectively, than those in the surface layer of plowed soil. Biomass and PMN levels were greatest in the surface 0 to 7.5-cm layer of no-tillage soil and decreased with depth in soil to 30 cm. Biomass and PMN levels of plowed soil, however, were generally greatest at the 7.5 -15 cm depth. Microbial biomass levels were closely associated with soil distributions of total C and N, water content, and water-soluble C as influenced by tillage management. Potentially mineralizable N levels in soil were primarily associated with distributions of microbial biomass and total N. Absolute levels of PMN and microbial biomass and the relative differences with tillage management were dependent on climatic, cropping, and soil conditions across locations. The additional N contained in soil biomass and PMN in the surface 0-7.5 cm of no-tillage compared with plowed soils ranged from 13 to 45 and 12 to 122 kg N/ha, respectively, for 6 of 7 locations. Fertilizer placement below the biologically rich surface soil layer and/or rotational tillage may improve short-term nitrogen use efficiency and crop growth on reduced-tillage soils.

Using water hyacinth [Eichhornia crassipes (Mart.) Solms] for wastewater renovation produces biomass that must be disposed of. This biomass may be anaerobically digested to produce CH₄ or added to soil directly as an amendment. In this study, fresh and anaerobically digested water hyacinth biomass, with either low or high N tissue content, were added to soil to evaluate C and N mineralization characteristics. The plant biomass was labeled with ¹⁵N before digestion. The fresh plant biomass and digested biomass sludge were freeze-dried and ground to pass a 0.84-mm sieve. The materials were thoroughly mixed with a Kindrick fine sand (Arenic Paleudults) at a rate of 5 g kg⁻¹ soil and incubated for 90 d at 27 °C at a moisture content adjusted to 0.01 MPa. Decomposition was evaluated by CO₂ evolution and ¹⁵N mineralization. After 90 d, approximately 20% of the added C of the digested sludges had evolved as CO₂ compared to 39 and 50% of the added C of the fresh plant biomass with a low and high N content, respectively. First-order kinetics were used to describe decomposition stages. Mineralization of organic ¹⁵N to ¹⁵NO™₃-N accounted for 8% of applied N for both digested sludges at 90 d. Nitrogen mineralization accounted for 3 and 33% of the applied organic N for fresh plant biomass with a low and high N content, respectively.

A biomass productivity model and a soil loss model were used to simulate effects of mining and erosion on the productivity potential of a 600-ha site. Biomass productivity, expressed as a relative productivity index pi (Full-size image (<1 K)), was computed as a product of a root distribution function and limiting soil property levels derived from literature. Distribution of soil loss or deposition was estimated using a recently developed erosion-deposition model. Effects of two scenarios are compared. In the first, the site is assumed to have been surface mined for coal and the effects on productivity are examined if existing soil is replaced with a minesoil. In the second, erosion-deposition model is used to predict changes in productivity after a severe storm. Under the assumptions of this study, biomass productivity appears more likely to decline because of mining than because of erosion.

We examined the effects of water supplementation and nitrogen amendment on biomass, cover, and density of annual plants on a termite—free and a termite—present area in the Chihuahuan Desert. Soil moisture was higher in the termite than in the termite—free plots, and in the watered than in the unwatered plots during the spring and summer. There were no differences in soil moisture among plots during the winter. Soil nitrogen was higher in the termite—free than in the termite plots. There were no differences in total plant biomass produced in termite and termite—free areas. There were significant differences in relative abundances of species among treatments. natural rainfall was sufficient for maximum spring—annual biomass development on all plots except for the termite—free unfertilized, unwatered plots. These were the driest plots but had high soil nitrogen. Most of the herbaceous species responded to the water amendments by lengthening growing seasons, increasing density, or increasing biomass. When there was sufficient water for most of the spring annuals, high soil nitrogen levels favored increased densities and biomasses of Descurainia pinnata and Lepidium lasiocarpum. The absence of C₄ summer annuals in the high—nitrogen plots suggests that relatively high soil nitrogen adversely affected the summer annuals. Termite—free watered plots had higher soil moisture than the termite—unwatered plots, but summer annuals were relatively abundant on the latter. Water amendments had a greater effect on the species abundances in the termite—free area than in the one with termites. In the area with termites, nitrogen amendments had a greater effect on species abundances. Species diversity and richness were affected by fertility as was species composition. This study demonstrates that we must understand patterns of soil nitrogen availability and processes affecting nitrogen availability in addition to water availability, in order to understand productivity and species composition of Chihuahuan Desert annual plants.

The computer model NCSOIL computes the dynamics of C and N transformations in soil. In its original version, the microbial biomass assimilated organic N from the decay of organic pools, and inorganic N was immobilized only when decomposed organics were deficient in N with respect to biomass needs for growth (NCSOIL—direct version). A new version was presented in which it was assumed that microbial biomass immobilized N only from the inorganic pool, while the decay of organic N always resulted in N mineralization (NCSOIL-MIT version). NCSOIL-MIT was calibrated and tested for the incorporation of inorganic ¹⁵N in the organic soil N against experimental data and compared with the direct version. Net N mineralization was highly sensitive to the microbial efficiency of soil decomposable organic C incorporation. An efficiency of 0.2 gave a good account of net N mineralization for soils unamended with organic substances. NCSOIL-MIT simulated the incorporation of ¹⁵N, added as NH₄, into the organic soil pools. The kinetics of ¹⁵N immobilization was sensitive to microbial efficiency as well as to other parameters that control soil N turnover. NCSOIL—direct version required the decomposition of polysaccharide-like material in order to obtain ¹⁵N incorporation in the soil organic pools, and the kinetics of ¹⁵N immobilization depended on the rate of polysaccharide decomposition.

Experiments during three rice croppings to determine the possibility of introducing a non-indigenous blue-green alga (BGA) in a soil already with an indigenous species showed that the indigenous BGA established its growth better than did the introduced species. Application of the BGA, either surface-applied or incorporated in soil, did not show any significant effect in the total yield of algae biomass. Orthophosphate in floodwater and total solar radiation showed significant positive relationships with algae biomass. Addition of urea fertilizer suppressed the growth of BGA. Maximum algal biomass was higher in the dry than in the wet season prior to maximum tillering of rice or at about 60 days after transplanting.

Organic amendments were added to a southwestern United States forest nursery sandy loam soil to determine the effects on soil nutrient reserves and subsequent growth of 1.5+0 ponderosa pine (Pinus ponderosa Laws.) seedlings. Treatments included irradiated sewage sludge, peat moss and pine bark each at 67 t/ha, sawdust at 43 t/ha, and a control that received no organic matter. Sludge caused immediate increases in soil nutrients, especially N and P. Sawdust resulted in near complete N immobilization 45 d after application. Peat moss and bark did not significantly alter soil nutrients. All treatment effects disappeared within 6 months of application. Amendments did not significantly alter seedling survival, biomass or yield (caliper ⩾ 3 mm). Seedling biomass was positively correlated with early soil nutrient status, but growth was not significantly improved. The modest, short-term nutritional benefits indicate single applications of organic amendments are ineffective in improving the nutrient status of sandy nursery soils of the Southwest.