Use of tracers to determine the dynamic nature of organic matter, The

Includes bibliographical references (pages 31-43).Early experiments with 13C, 14C and 15N established the high rate of internal cycling of soil organic matter and reintroduced the concept of an active and passive phase in soil humus turnover. Later studies confirmed non-tracer investigations indicating that the percent decomposition of added materials is relatively independent of the rates of addition but dependent on its form and composition. The initial decomposition rate, plus the stabilization of microbial products in soil, must be taken into account when interpreting degradation of 14C enriched straw, roots, microbial tissue and specific components or in carbon dating naturally occurring 14C. Where initial decomposition data could be described by first order kinetics we calculated decay rate constants with and without the consideration of biosynthesis. Decay rates for laboratory systems were twice those for tropical field soils and eight times those calculated for temperate climates. The data were used in a model incorporating the concepts of microbial biosynthesis and recalcitrant and decomposable soil organic fractions which can both be physically protected. This realistically described the behaviour of soil-C in a Canadian grassland before and after cultivation

Carbon allocation, belowground transfers, and lipid turnover in a plant–microbial association

Includes bibliographical references (pages 1622-1623).Radioactive tracers were used to study the C allocation to coarse and fine roots, aboveground plant tissues, mycorrhizal lipids, belowground respiration, and soil in a mycorrhizal association. Sorghum bicolor (L.) Moench was grown in soil with a nonmycorrhizal microbial inoculum with and without Glomus clarum, a mycorrhizal inoculant. Fifty-one-day-old mycorrhizal (M) and nonmycorrhizal (NM) plants were subjected to a 3-h exposure to 14CO2 and sequentially harvested after 52, 54, 57, 64, and 76 d. Mycorrhizal plants assimilated 21% more 14C than NM plants, even though they were slightly smaller in size. They also had a higher percentage and absolute allocation of 14C to root tissue, belowground respiration, and soil. Mycorrhizal roots had a higher content of total lipids and total fatty acids. The fungal fatty acid 16:1ω5, usually associated with arbuscular mycorrhizal fungi, comprised up to 29.5% of the total fatty acid content of M roots, while NM roots had only trace levels of this molecule. Thin-layer chromatography was used to separate the fatty acids extracted from the roots. The 14C of the various components was determined by radiography. The 14C mean residence time (MRT) of the mycorrhizal fatty acid 16:1ω5 was calculated at 7.1 d. The monoenoic, saturated, and total fatty acids had MRTs ranging from 11.1 to 14.3 d. The lipids of NM roots incorporated less 14C label. This underscores the difference in the lipid C cycle between the M and NM roots. Translocation of the 14C to soil was 6.3% of the photosynthesized C in the M plants relative to only 2.4% in the NM plants, giving an indication of its movement into the mycorrhizal hyphae as well as to the soil

Response of mycorrhizal and P-fertilized soybeans to nodulation by Bradyrhizobium or ammonium nitrate

Includes bibliographical references (page 150).Management of N2-fixing bacteria or P-scavenging endomycorrhizae may lead to decreased fertilizer use on extensively cropped lands. To measure the effectiveness of these microsymbionts, soybean [Glycine max (L.) Merr. cv. Amsoy 71] plants were grown in a growth chamber in a soil [Josephine silty clay loam (mesic Typic Haploxerult)] low in plant-available N and P. Plants were inoculated with different Bradyrhizobium strains or received nutrient solutions of different N concentrations (0.0, 1.0, 2.0, 4.0 mM N) and P adequate for maximum plant growth under these conditions. Other plants were infected with a vesicular-arbuscular mycorrhizal (VAM) fungus and a Bradyrhizobium strain and received no N or P in the nutrient solution. The purpose of this study was to determine the growth response of soybean to N fertilization or nodulation by B. japonicum under conditions of high P availability or V AM-assisted P uptake. Nodulated non-VAM soybean plants had dry weights and development similar to that of the 4.0 mM N fertilizer treatment. Total N and Mn, leaf area, and leaf P of nodulated plants were higher than in the comparable N-fertilized plants in the absence of P stress. Soybeans infected with both the VAM fungus and Bradyrhizobium were similar in total dry weight, leaf area, and development to plants that received 1.0 or 2.0 mM N. They, however, contained more leaf N, more root Cu and Zn, and less Mn and P than the 2.0 mM N treatment. It is concluded that a number of host characteristics of nodulated plants are due to the altered functional aspects of the symbiosis and not N input alone. The presence of the VAM fungus can decrease nutrient stress in environments limited in P, Zn and Cu, elements essential in N2 fixation

Comparisons between P-fertilized and mycorrhizal plants

Includes bibliographical references (pages 155-156).In experimentation with vesicular-arbuscular mycorrhizal (VAM) fungi, the availability of non-VAM control plants of equal size to VAM plants is a fundamental requirement. The purpose of this work was to determine nutrient regimes needed to achieve growth equivalence between VAM and non-VAM plants. Soybean [Glycine max (L.)Merr.] cv. Amsoy 71 and sorghum [Sorghum bicolor (L.) Moench] cv. Bok 8 plants were grown under controlled conditions in a soil (Josephine silty clay loam, mesic Typic Haploxerult) low in plant-available P. Soybeans were inoculated with one of four species and sorghum with one of two species of VAM fungi. Non-inoculated control plants received nutrient solutions that contained 0.0, 0.2, 0.4, or 1.0 mM P. while the growth of P-supplemented controls may be equivalent to VAM plants, an important question remains: Are these plants also equivalent in terms of such functional parameters as leaf development, dry matter partitioning, and nutrient assimilation? The objective of this experiment was to answer these questions. The response to VAM colonization was similar in both hosts, although less extensive colonization was observed in sorghum. Dry weight, leaf area, and P content increased exponentially with nutrient solution P level. Plants colonized with VAM fungi grew 3 to 6 times larger than the P-free controls but attained only 35 to 65% of maximum growth possible with high fertilizer P input. Host response to VAM colonization was equivalent to that of plants receiving between 0.12 and 0.22 mM P for phytomass, leaf area, and N content. Mycorrhizal plants contained less P, Mn, and root Fe but more Zn and Cu than comparable plants fertilized with P. It was concluded that P-treated, non-VAM plants differed physiologically and anatomically from VAM plants of equivalent size grown under P stress. It may therefore be necessary to establish the comparability of VAM plants and of "VAM-equivalent controls" separately for each plant parameter of interest. Even then, differential growth responses in VAM-host associations may prevent complete comparability between VAM and P-fertilized plants

Interpretation of soil carbon and nitrogen dynamics in agricultural and afforested soils

Includes bibliographical references (pages 1627-1628).Interpretation of soil organic C (SOC) dynamics depends heavily on analytical methods and management systems studied. Comparison of data from long-term corn (Zea mays)-plot soils in Eastern North America showed mean residence times (MRTs) of SOC determined by 14C dating were 176 times those measured with 13C abundance following a 30-yr replacement of C3 by C4 plants on the same soils. However, MRTs of the two methods were related (r 2 = 0.71). Field 13C MRTs of SOC were also related (R 2 = 0.55 to 0.85) to those measured by 13CO2 evolution and curve fitting during laboratory incubation. The strong relations, but different MRTs, were interpreted to mean that the three methods sampled different parts of a SOC continuum. The SOC of all parts of this continuum must be affected by the same controls on SOC dynamics for this to occur. Methods for site selection, plant biomass, soil sampling and analysis were tested on agricultural, afforested-agriculture, and native forest sites to determine the controls on SOC dynamics. Soil-C changes after afforestation were −0.07 to 0.55 Mg C ha−1 yr−1 on deciduous sites and −0.85 to 0.58 Mg C ha−1 yr−1 under conifers. Soil N changes under afforestation ranged from −0.1 to 0.025 Mg N ha−1 yr−1 Ecosystem N accumulation was −0.09 to 0.08 Mg N ha−1 yr−1 Soil C and N sequestration but not plant biomass were related to soil Ca, Mg, and K contents. Comparative, independent assays of long-term plots provides information for concept testing and the confidence necessary for decision-makers determining C-cycle policies

Controls on soil carbon sequestration and dynamics: lessons from land-use change

Includes bibliographical references (pages 82-83).Soil carbon (C) dynamics and sequestration are controlled by interactions of chemical, physical and biological factors. These factors include biomass quantity and quality, physical environment and the biota. Management can alter these factors in ways that alter C dynamics. We have focused on a range of managed sites with documented land use change from agriculture or grassland to forest. Our results suggest that interactions of soil type, plant and environment impact soil C sequestration. Above and below ground C storage varied widely across sites. Results were related to plant type and calcium on sandy soils in our Northern sites. Predictors of sequestration were more difficult to detect over the temperature range of 12.4°C in the present study. Accrual of litter under pines in the moist Mississippi site limited C storage in a similar manner to our dry Nebraska site. Pre-planting heterogeneity of agricultural fields such as found in Illinois influences C contents. Manipulation of controls on C sequestration such as species planted or amelioration of soil quality before planting within managed sites could increase soil C to provide gains in terrestrial C storage. Cost effective management would also improve soil C pools positively affecting soil fertility and site productivity.Publisher version: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3380508

Applicability of the carbon-dating method of analysis to soil humus studies

Includes bibliographical references.The organic fraction of soil is known to be composed of the soil biomass, partially decomposed plant and animal residues, and the materials commonly referred to as humic substances. Knowledge of the persistence of these fractions in soil is vital to the understanding of their contribution to soil fertility and soil genesis. Much information concerning the biochemistry of the humus materials also could be obtained through a knowledge of the mean residence times of the various organic fractions.This is a non-final version of an article published in final form in Soil Science 104, no. 3 (September 1987): 217-224. Publisher version: http://journals.lww.com/soilsci/Citation/1967/09000/APPLICABILITY_OF_THE_CARBON_DATING_METHOD_OF.10.aspx

Changes in ecosystem carbon following afforestation of native sand prairie

Includes bibliographical references (pages 1622-1624).Determining the dynamics of carbon (C) as a function of vegetation and residue inputs is important for predicting changes in ecosystem functions and the global C cycle. Litter and soil samples were analyzed from plantations of eastern red cedar (Juniperous virginiana) and ponderosa pine (Pinus ponderosa) and native prairie at the Nebraska National Forest to evaluate the impact of different types of land management on soil C contents and turnover rates. Total soil C to a depth of 1 m was greatest in the cedar stands. Pine ecosystems stored more C in the tree biomass and litter but lost more native prairie C from the soil. The soil 13C content showed 82% of the original, and prairie C remained under cedars compared with ∼45% under pine. Soil cation contents were greatest overall in cedar soils and lowest in pine. The C content in cedar soils was strongly related to Ca content. Differences in microbial community fatty acid profiles were related to vegetation type, and nutrients explained ∼60% of the variation in profiles. Our research indicates that changes in soil C and nutrient content following conversion from prairie to forest are dependent on tree species planted, characteristics of the plant litter, and cation cycling in the plant–soil system

Biological and molecular structure analyses of the controls on soil organic matter dynamics

Includes bibliographical references (page 170).The dynamics of soil organic carbon (SOC) are controlled by the interaction of biological, physical, and chemical parameters. These are best measured by a combination of techniques such as long-term field sites with a C3↔C4 plant switch. Acid hydrolysis and 14C- dating measure the mean residence time (MRT) of the resistant fraction. Long-term incubation allows the in situ biota to identify and decompose the labile SOC components. Statistical analysis (curve fitting) of the CO2 release curves, determines the pool size and of the two labile fractions (1). The effect of chemical structure is measured with pyrolysismolecular beam mass spectrometry (py-MBMS). The dynamics of charcoal, clay and silt are measured with both 13C and 14C

Visions of a more precise soil biology

Includes bibliographical references (pages 389-390).Soils have often been viewed as a black box. Soil biology is difficult to study with the precision we would wish, due to the presence of considerable soil heterogeneity, a huge diversity of organisms, and a plethora of interacting processes taking place in a complex physical-chemical environment. We have isolated a tiny fraction of the known organisms, and the possible interactions of soil parent materials, landscape, land use, depth and time with the biota mean that we are to some extent still fumbling in the dark. There have been great advances, but we argue that the pace of advance could be faster. To progress, science needs new theory and concepts but also acceptable methodologies. Coherent and generally accepted theoretical knowledge exists in many areas, but there is a shortage of valid and exact methods to test new and sometimes even old hypotheses. New methods add knowledge, but they also can add to the confusion if they are not tied to the existing knowledge base. We speculate on how to improve soil biology through improving the way we perform and interpret research. Can we deal with soil variability? Can we measure the critical variables with adequate precision to test our hypotheses? Can we avoid reinventing the wheel? Can we find a balance between the freedom to test new and maybe even controversial ideas and the control and direction of research required by society?