Search

The dynamics of C in terrestrial ecosystems can be traced on the basis of the marked differences in the δ13C signatures of plants possessing the C3, C4or CAM photosynthetic pathways. When two C sources differing in natural 13C abundance (δ13C) contribute to a mixture, the relative contribution of each can be quantitatively apportioned by the use of a two-end-member mixing model. This ability of δ13C to partition source is unique among stable isotopes important in the biosphere, where with few exceptions, natural abundance signatures are used as qualitative rather than quantitative tracers because of isotopic fractionation. The many and varied applications of δ13C in apportioning C source are examined herein. They include estimation of soil organic matter turnover due to either a known or presumed change in vegetation cover, the contribution of organic materials to the soil C pool, the contribution of C source to vertebrate and invertebrate diets, disentangling roots in mixed C3–C4stands, and partitioning the sources of respired CO2in various C3–C4mixtures, i.e., between plants and soil, between plants, between animal excreta slurries and soil, within fungal media and within biochar-amended soil.

The increased demand for organic farm products, and the attendant restrictions on the use of synthetic N fertilizers in both traditional and organic agriculture, has focused attention on the efficacy of organic sources of N, including animal excreta. However, questions arise in regard to the agronomic and environmental benefits of excreta as alternative sources of N. These questions are best addressed through the experimental use of 15N-enriched excreta, which enable the labeled source to be unequivocally traced through the soil-plant-atmosphere continuum, which N difference or N equivalence techniques cannot achieve. In contrast to organic and inorganic N compounds that can be purchased commercially with a specified 15N label on the component N moieties, 15N-enriched excreta must be prepared by the investigator. The methods of production and the uniformity of labeling of the components of 15N-enriched excreta (feces, urine) and their admixture (slurry), the N use efficiencies of excreta in crop production, and comparisons and interactions of excreta with synthetic N sources are reviewed. Losses of N from excreta-amended soils to the environment and the residual N value of excreta in crop sequences are also examined. It was concluded that while similar agronomic and environmental issues surround the use of both synthetic fertilizers and excreta as sources of N for plant nutrition, the processes differ in intensity and duration both spatially and temporally.

To determine the effects of urban sewage biosolids on the symbiotic effectiveness of Rhizobium leguminosarum bv. trifolii and N2 fixation, glasshouse and laboratory studies were carried out with several soils, biosolids, and biosolid application levels. Symbiotic effectiveness of R. l. trifolii was estimated as the dry weight or N content of seedlings of subterranean clover grown with only N2 fixation and seed N as the available nitrogen sources. The N fixed by legumes in unamended and biosolid-amended soils was determined using the 15N isotope dilution method. Six soils were represented in the experiments. Each of these was equilibrated over a period of 12 months with dried, finely ground biosolids (DWS) from the Malabar sewage treatment plant, at biosolids levels ranging from the equivalent of 60 to 240 t DWS/ha. One of the soils was also equilibrated with each of 4 other biosolids. The maximal concentration of heavy metals in soil amended with biosolids was 1026 mg/kg. The effect of biosolids on symbiotic effectiveness depended on the soil type and biosolid applications level. Thus, biosolids reduced the symbiotic effectiveness of R. l. trifolii in 2 of the 6 soils, although at different levels of biosolid. In most soil treatments N2 fixation was detected in subterranean clover, confirming the persistence of symbiotically effective rhizobia in most biosolids-amended soils. In addition, in strongly acidic soils plant N and N2 fixation increased significantly with biosolids addition, probably in response to higher soil pH, exchangeable Ca, and available P. In the treatments in which the symbiotic effectiveness of R. l. trifolii was reduced by biosolids, this was reflected in poor N2 fixation. However, symbiotic effectiveness did not correlate well with N2 fixation, probably because increases in soil nitrate at higher biosolids levels inhibited N2 fixation. Nevertheless, there were instances at 240 t DWS/ha where this was unlikely to explain the decrease in N2 fixation. It was concluded that adverse effects of biosolids on symbiotic effectiveness depend first on soil type, and then on biosolid type and application level; and the response in symbiotic effectiveness to adding biosolids to soil needs to be determined for each distinctively different site of biosolids reuse. Additional keywords: symbiosis, legume, heavy metal.

After decades of searching for a rapid method to estimate the N mineralization capacity of soil, there is still no consistent recommendation. It is legitimate to examine the causes for the often‐conflicting results in literature. The efficacy of various references that have been used as benchmarks for assessing chemical and physical indices in the literature is critically reviewed in this paper. Gross N mineralization and consumption during waterlogged and aerobic incubations were estimated in a wide range of soils. It was found that equivalent to 17 to 90 and 23 to 59% of the mineralized N was consumed during the waterlogged and aerobic incubations, respectively. As net N production rate represents the balance between N‐producing and N‐consuming processes, it appears difficult to find a simple method that could be used to predict the net effect of several concurrent processes. We used the gross N mineralization as a reference criterion for N mineralization ability. Total organic N, water‐soluble organic N, alkali‐hydrolyzable N, acid‐hydrolyzable N, hot salt‐hydrolyzable N and N in the light organic matter fraction were assessed against this reference criterion. All indices except light fraction N were significantly related to gross N mineralization. Water‐soluble organic N had the highest correlation of all the indices tested. None of the chemically hydrolyzed N fractions consistently showed closer relationships with N mineralization than total organic N, suggesting that these chemical methods are ineffective in extracting a biologically labile fraction of soil organic N.