N. G. Juma

Simulation of carbon and nitrogen transformations in soil: Mineralization

Abstract If mathematical models of decomposition and transformation processes are to be rigorously validated, they should be tested against the microbial dynamics from which these processes arise. A mathematical model was constructed from kinetic equations for microbial activity reported in the literature and in earlier models, and was tested against C and N mineralization during incubation of labelled glucose, cellulose and crop residue on several soils. The model was able to reproduce temporal trends in the mineralization, immobilization and retention of labelled C and N to within 10% of recorded values over time scales of hours, days and years following soil amendments. By treating the humification and adsorption of microbial products as functions of soil clay content, the simulated mineralization of C amendments was reduced, and retention increased, to extents consistent with recorded data in soils with clay contents which varied from 4 to 34%. By allowing the decomposition of all soil organic matter to be determined by the total concentration of microbial biomass, the model also reproduced most of the increased mineralization of non-labelled C recorded from 14C-amended soils.

Interrelationships between soil structure/texture, soil biota/soil organic matter and crop production

Abstract Interrelationships between soil structure/texture, soil biota/soil organic matter and crop productivity occur from a scale of micrometres to metres. In this paper an attempt has been made to integrate and quantify these interrelationships by using the themes of C and N mineralization and porosity using data from soils in western Canada. Although the concept of soil porosity (texture and structure) has been used to describe water and air movement in soil, it has not been fully explored to describe microbial, faunal and plant interactions. Future work has to incorporate the effects of soil porosity on biological processes in agro-ecosystems.

Simulation of carbon and nitrogen transformations in soil: Microbial biomass and metabolic products

The mineralization of C and N in soils involves the dynamic behaviour of the microbial biomass. This behaviour was reproduced in a simulation model which was used to study microbial growth and mineralization following soil amendments with labelled glucose and crop residues. Temporal trends of simulated microbial C and N associated with these amendments were consistent with those of labelled C and N estimated from chloroform fumigation (SD 1.0 to <0.01 gg−1 day−1 that were caused by changes in substrate availability.

Influence of texture on habitable pore space and bacterial-protozoan populations in soil

SummarySoil texture affects pore space, and bacterial and protozoan populations in soil. In the present study we tested the hypothesis that bacteria are more protected from protozoan predation in fine-textured soils than in coarse-textured soils because they have a larger volume of protected pore space available to them. The experiment consisted of three sterilized Orthic Black Chernozemic soils (silty clay, clay loam, and sandy loam) inoculated with bacteria, two treatments (with and without protozoa), and five sampling dates. The soils were amended with glucose and mineral N on day 0. On day 4 bacterial numbers in all three soils were approximately 3×109 g−1 soil. The greatest reduction in bacteria due to protozoan grazing occurred between day 4 and day 7. Compared to the treatment without protozoa, bacteria in the treatment with protozoa were reduced by 68, 50, and 75% in the silty clay, clay loam, and sandy loam, respectively. On day 4, 2 days after the protozoan inoculation, all protozoa were active. The numbers were 10330, 4760, and 15 380 g−1 soil for the silty clay, clay loam, and sandy loam, respectively. Between day 4 and day 7, the period of greatest bacterial decline, total protozoa increased greatly to 150480, 96160, and 192100 g−1 soil for the three soils, respectively. Most protozoa encysted by day 7. In all soils the addition of protozoa significantly increased CO2−C evolution per g soil relative to the treatment without protozoa. Our results support the hypothesis that bacteria are more protected from protozoan predation in fine-textured soils than in coarse-textured soils.

Nitrogen fixation efficiency, interspecies N transfer, and root growth in barley-field pea intercrop on a Black Chernozemic soil

SummaryBarley-field pea intercrops have been shown to increase N yield when grown under cryoboreal subhumid conditions. In this study, we extended previous research by testing the hypotheses that (1) the intercropped field pea fixes a greater proportion of its shoot and root N than does sole-cropped field pea; (2) N is transferred from the annual legume to the cereal during the growing season; and (3) root production is greater under intercropped than sole-cropped conditions. Unconfined microplots seeded to barley, field peas, or a barley-field pea intercrop were fertilized with N at 10 kg ha-1 as (NH4)2SO4 (5.21 atom % 15N excess). Both the intercropped and sole-cropped barley derived more than 93% of their N from the soil. In contrast, 40% of N in the intercropped field pea was derived from soil. This study provided no evidence for transfer of N from the legume to the cereal. On average, the proportion of N derived from air by both pea intercrops was 39% higher than that derived by the sole-cropped pea. Root length determined by a grid intersection method following digitization using an image analyzer tended to be higher under intercropping than in sole crops. We conclude that even on fertile soils benefits may accrue from annual intercropping that includes a legume. The benefits arise from (1) increased N production, (2) greater N-fixation efficiency, and/or (3) more shoot and root residue-N mineralization for subsequent crops.

Decomposition and Nutrient Cycling in Agro-Ecosystems

Agriculture manipulates energy fluxes, nutrient dynamics and hydrologic cycles. Descriptions of nutrient cycling and decomposition in agro-ecosystems are site-specific and thus generalizations are of limited validity unless processes and regulatory mechanisms are considered. Consequently, this chapter discusses organic matter status of agricultural soils, some of the mechanisms regulating processes and recently developed ideas on nutrient dynamics in manipulated soil-plant systems.

Performance of conventional and alternative cropping systems in cryoboreal subhumid central Alberta

A 3-year field study (1986-88) was conducted in central Alberta to discover how diverse soil-plant systems function under cryoboreal subhumid conditions. Barley, fescue, faba (field) bean and a barley/field pea intercrop were grown continuously on different soils at Ellerslie and Breton using two distinct tillage methods. The agronomic performance, weed-crop interactions and below-ground productivity of these cropping systems were examined (.)

Effect of glucose amendment on microbial biomass, spelling fertilizer 15N-recovery and distribution in a barley-soil-system

SummaryOne way to conserve fertilizer N in the plant-soil system is to immobilize it at the time of application by adding a readily available C source and to rely on the microorganisms to remineralize it to meet crop N demand during the season. The present study was conducted to determine the effects of microbial activity due to glucose amendment at the time of fertilization and planting on the distribution of fertilizer 15N at harvest among various N pools. Glucose C (150 g m-2) was added to soil at Ellerslie (Black Chernozem) in central Alberta at the time of seeding and fertilization with urea-15N (7.5 g m-2). Barley shoot mass, root mass, and root N at harvest in the non-glucose treatment were 1.8-fold, 1.9-fold, and 2.2-fold greater, respectively, than in the glucose treatment. The recovery of 15N in the soil-plant system was greater in the glucose (82%) than the non-glucose treatment (50%). Likewise, the recovery of 15N in soil was greater in the glucose treatment (72%) than the non-glucose treatment (22%). In both treatments most soil 15N remaining at the time of harvest was present as non-microbial organic 15N, but recovery of 15N in this pool was 3.4-fold greater in glucose-treated than in non-glucose-treated soil. The microbial response to the glucose addition effectively conserved fertilizer N in the active N phase; however, significant remineralization did not occur to meet plant N demands. Microbial transformations in the soil resulted in a constant ratio of non-microbial organic N formed per unit of microbial N formed and this ratio was not affected by the C amendments.

Shoot, root, soil and microbial nitrogen dynamics in two contrasting soils cropped to barley (Hordeum vulgare L.)

SummaryDynamics of barley N, mineral N, and organic N were compared at Ellerslie (Black Chernozem, Typic Cryoboroll) and Breton (Gray Luvisol, Typic Cryoboralf) in central Alberta, using 15N-urea. On average, shoot N and shoot 15N recoveries at Ellerslie (14.1 g m−2, 36%) were greater than at Breton (4.5 g m−2, 17%). Root N (g m−2) did not significantly differ between sites (0–30 cm) but root 15N recovery was greater at Breton (3.4%) than Ellerslie (1.8%). Low levels of shoot N and shoot 15N at Breton were partly due to very wet soil conditions in July, which resulted in premature shoot senescence and low plant N uptake. Although the total 15N recoveries from the system (to 30 cm depth) at Ellerslie (63%) and Breton (56%) were similar, soil 15N was greater at Breton (35%) than at Ellerslie (26%). There were no differences in mineral N between sites but the average 15N recovery in the mineral-N pool was significantly greater at Ellerslie (3.3%) than at Breton (1.6%). There was no difference in 15N recovery in the microbial biomass (∼3%) between sites, although non-microbial organic 15N was greater at Breton (31 %) than at Ellerslie (20%). The two soils showed differences in the relative size of kinetically active N pools and in relative mineralization rates. Microbial N (0–30 cm) was greater at Ellerslie (13.3 g m−2) than at Breton (9.9 g m−2), but total microbial N made up a larger proportion of total soil N at Breton (1.6%) than at Ellerslie (0.9%). In the 0–10 cm interval, microbial N was 1.7-fold greater and non-microbial active N was 3-fold greater at Breton compared to Ellerslie, when expressed as a proportion of total soil N. Net N mineralization in a 10-day laboratory incubation was 1.4-fold greater in the Black Chernozem (0–10 cm interval) from Ellerslie, compared to the Gray Luvisol from Breton, when expressed per gram of soil. Net N mineralization in the soil from Breton was double that of the soil from Ellerslie, when expressed as a proportion of soil N. Although soil N (g m−2) was 2.5-fold greater at Ellerslie compared to Breton, it was cycled more rapidly at Breton.

Management effects on the dynamics and storage rates of organic matter in long-term crop rotations

Factors controlling soil organic matter (SOM) dynamics in soil C sequestration and N fertility were determined from multi-site analysis of long-term, crop rotation experiments in Western Canada. Analyses included bulk density, organic and inorganic C and N, particulate organic C (POM-C) and N (POM -N), and CO2-C evolved during laboratory incubation. The POM-C and POM-N contents varied with soil type. Differences in POM-C contents between treatments at a site (δPOM-C) were related (r2= 0.68) to treatment differences in soil C (δSOC). The CO2-C, evolved during laboratory incubation, was the most sensitive indicator of management effects. The Gray Luvisol (Breton, AB) cultivated plots had a fivefold difference in CO2-C release relative to a twofold difference in soil organic carbon (SOC). Soils from cropped, Black Chernozems (Melfort and Indian Head, SK) and Dark Brown Chernozems (Lethbridge, AB) released 50 to 60% as much CO2-C as grassland soils. Differences in CO2 evolution from the treatment with the low...