Net mineral-N Production Research Articles

Does low N-mineralisation rate in soils under grass help limit nitrate leaching in summer?

This paper aimed to confirm the hypothesis that rates of ammonification and net mineral-N production in soils under grass in summer are low and this, rather than nitrate uptake by plants, reduces mineral leaching from soils in summer. Six sets of soil samples were collected from under mown grass on the University of York campus in the UK. Samples were taken from two depths in late summer (August) and late autumn (November) to compare seasonal differences in N species transformations when field-moist soils were incubated for a week at ambient outdoor temperatures after prior removal of vegetation. Ammonification and net mineral-N production rates were low in August in spite of warm temperatures. Net mineral-N production rates were also low in November. The results agreed with those of an earlier study in a different year after considering weather differences between years. They support the hypothesis that litter accumulated over autumn/winter will be minimally mineralised and retain N before the temperature rises in spring. The study shows the merit of measuring concentrations of mineral-N species in both fresh soils and soils incubated at ambient outdoor temperatures after removing plant material to eliminate plant N uptake effects. The results suggest that soil C% and N% are more important than soil C:N ratio alone in understanding controls on N transformations.

Elevated [CO2] effects on herbage production and soil carbon and nitrogen pools and mineralization in a species-rich, grazed pasture on a seasonally dry sand

Rising concentrations of atmospheric [CO2] in a multi-species ecosystem can influence species composition and increase plant productivity, but have a less predictable effect on soil C storage and nutrient availability. Using a free-air [CO2]-enriched (FACE) system and seasonal sampling over a 5-year period, we examined the influence of elevated atmospheric [CO2] (475 μL L−1) on soil C and N pools and mineralization in a fertilized (P, K, S), sheep-grazed pasture of mixed grass, clover, and forb species on a seasonally dry sand (Mollic Psammaquent). Annual yields of herbage dry matter ranged from about 300 to 1600 g m−2. Total yields did not increase significantly under elevated [CO2], but the proportions of clovers and forbs increased markedly. Most properties in 0–50 mm-depth soil differed significantly (P 0.10) for moisture, pH, total C and N, extractable C and organic N, microbial C, and mineral-N. However, microbial N, CO2-C production (0–14 days) in field-moist soil, and net mineral-N production (14–56 days) in soil at 60% of water-holding capacity were significantly higher (per unit weight of soil) in the elevated-[CO2] treatment (P=0.071, 0.063, 0.003, respectively); the degree of these treatment differences was roughly similar when values were also expressed on a total C or N basis. Relationships with soil moisture were mainly non-significant for microbial C and N, but mainly significant (P<0.05) for net mineral-N production in field-moist soil, and highly significant (P<0.001) for CO2-C production. Overall, the data tend to suggest greater soil metabolic activity, but little if any change in soil C pools, after 5 years' exposure of the pasture to elevated [CO2]. They do, however, suggest increased availability of N, probably because of increased inputs from N-fixing clovers.

Limitations to carbon mineralization in litter and mineral soil of young and old ponderosa pine forests

Summer drought is a feature of the semi-arid region of central Oregon, USA, where vegetation naturally develops into ponderosa pine ( Pinus ponderosa var. Laws) forest. Forest management consists of clearcut harvest and natural regeneration. Soil microbial activity is interconnected with forest processes because substrate quality and availability can be important driving variables. Stand development influences the soil water regime, and water availability may also limit microbial activity. We determined factors limiting litter and mineral soil carbon (C) mineralisation rates in undisturbed old growth and regenerating (hereafter, young) ponderosa pine stands under a semi-arid climate. Mass of litter and dead fine roots did not differ significantly between the stands, but litter substrate quality was different. Young stand litter had significantly higher concentrations of total nitrogen (N), extractable organic N, extractable C, and microbial C and N than that from the old stand, probably because of litter fall from the broadleaved shrub understorey, including the N-fixing species Purshia tridentata (Pursch) DC, that comprised 40% of the young stand’s leaf area. The old stand contained no understorey. For litter samples from the two stands, wetted to 60% of water-holding capacity (WHC), net mineral-N and CO 2–C mineralisation rates were similar despite the substrate quality differences. Mineral soil properties at 0–0.1 m depth were similar in the two stands, except for lower CO 2–C production in samples from the young stand; at 0.1–0.5 m depth, total C and N and microbial N concentrations were higher in the young stand. Net mineral-N production in field-moist soil, sampled during a typical summer drought and incubated at 25 °C for 56 days, was generally 3–6 mg kg −1 soil at both sites, but increased up to 29 mg kg −1 upon wetting to 60% of water-holding capacity. Over 56-day-long incubations, wetting also increased litter and soil microbial respiration rates by factors of about 500 and 3, respectively. The incubations yielded a proportionality between respiration rate and water content that was supported by in situ measurements of soil respiration in the young stand, before and after irrigation. A hypothetically wet year without soil water deficit caused a 2.5-fold increase in a modelled estimate of the young stand’s annual soil respiration rate. Litter and soil C mineralisation rates in these ponderosa pine forests thus appeared to be limited much more by the availability of water than by a lack of available C or N substrates.

Afforestation of pastures with Pinus radiata influences soil carbon and nitrogen pools and mineralisation and microbial properties

In New Zealand, Pinus radiata D. Don is frequently planted on land under pasture primarily for production forestry, but with the added advantage of potentially offsetting carbon dioxide (CO2) emissions from energy and industrial sources. Conversion of pasture to P. radiata plantations can, however, result in lowered contents of soil carbon (C) at some sites. We here examine the effects of this land-use change on soil C and nitrogen (N) pools, and on microbial properties involved in the cycling of these nutrients, at 5 paired sites, each with an established pasture and P. radiata plantation. Four sites had first-rotation trees aged 12–30 years and the other site second-rotation trees aged 20 years. In mineral soil at 0–10 cm depth, total and microbial C and N, extractable C, CO2-C production, and, generally, net mineral-N production were lower under P. radiata than under pasture; differences were significant (P &lt; 0.05), except for total and extractable C at 2 sites. Differences between these land uses were less distinct in soil at 10–30 cm depth. On an area basis, total C in 0–30 cm depth soil was lower under P. radiata than under pasture at most sites, but significantly lower at only one site. Total N, microbial C and N, and CO2-C and net mineral-N production were, however, again generally significantly lower under P. radiata. These ecosystem differences were less marked, although still present, except for CO2-C production, when forest litter (LFH material) was included in the area calculations. Overall, our study suggests that afforestation with P. radiata leads to a reduction in total N, microbial biomass, and microbial activity, but a less consistent effect on soil C storage after one rotation.

Decomposability of C3 and C4 grass litter sampled under different concentrations of atmospheric carbon dioxide at a natural CO2 spring

Elevated concentrations of atmospheric CO2 can influence the relative proportions, biomass and chemical composition of plant species in an ecosystem and, thereby, the input of litter nutrients to soil. Plant growth under elevated CO2 appears to have no consistent effect on rates of litter decomposition; decomposition can, however, differ in C3 and C4 plant material from the same CO2 environment. We here describe the decomposability of leaf litter of two grass species – the C3 Holcus lanatus L. (Yorkshire fog) and C4 Pennisetum clandestinum Hochst. (kikuyu) - from an unfertilized, ungrazed grassland at a cold CO2 spring in Northland, New Zealand. Decomposability was measured by net CO2–C production from litter incubated for 56 days at 25 °C in a gley soil from the site; net mineral-N production from litter was also determined. Both litter and soils were sampled under `low' and `high' concentrations of atmospheric CO2. Decomposition of H. lanatus litter was greater than that of P. clandestinum litter throughout the 56-day incubation. Decomposition tended to be greater in `high-CO2' than in `low-CO2' H. lanatus litter, but lower in `high-CO2' than `low-CO2' P. clandestinum litter; differences were, however, non-significant after 28 days. Overall, litter decomposition was greater in the `low-CO2' than `high-CO2' soil. Differences in decomposition rates were related negatively to litter N concentrations and positively to C:N ratios, but were not predictable from lignin:total N ratios. Net mineral-N production from litter decomposition did not differ significantly in `high-CO2' and `low-CO2' samples incubated in `low-CO2' soil; in `high-CO2' soil some net immobilization was observed. Overall, results indicate the likely complexity of litter decomposition in the field but, nevertheless, strongly suggest that rates of decomposition will not necessarily decline in a `high-CO2' environment.

Carbon and nitrogen pools and mineralization in a grassland organic soil at a New Zealand carbon dioxide spring

Recent evidence from naturally occurring CO2 springs suggests that soil organic C levels can increase under prolonged exposure to elevated atmospheric CO2. Here we describe C and N pools and metabolism in a strongly acid organic soil under different concentrations of atmospheric CO2 (averaging 543–729μl l−1 ca. 20cm above ground level at four sampling locations and 1220–3900μl l−1 at 10 others) in an unfertilized, ungrazed grassland at Hakanoa Springs in Northland, New Zealand. In 0–5cm depth soil, organic C (mean 274g kg−1) was similar in these two sets of samples, whereas total N was slightly higher (P>0.05<0.10) in the lower CO2 grouping. Extractable and microbial C and N, CO2-C and net mineral-N production, and their ratios with organic C or total N were also similar in the two sets of samples. These ratios were, however, all significantly higher than those in an adjacent gley soil under elevated CO2 at this site. Although organic C concentrations in this organic soil are unrelated to current concentrations of atmospheric CO2, the soil organic matter does appear to be enriched in labile C and N components.

Microbial processes in relation to carbon, nitrogen and temperature regimes in litter and a sandy mineral soil from a central Siberian Pinus sylvestris L. forest

The coniferous forests of Siberia contain a significant fraction of the world's terrestrial C. We have assessed organic matter storage and potential metabolic activities in one of these forest stands by determining C and N pools, microbial properties and C and N mineralization at different temperatures in litter and a sandy mineral soil (a Pergelic Cryochrept) from a 215-y old forest of Pinus sylvestris L. Relationships between CO 2–C production in the laboratory and field were also assessed. The site was near Zotino in central Siberia at 61° N, 89° E and has an average air temperature of ca. −4°C and an annual precipitation of ca. 620 mm; it is subject to water deficits during summer and to recurrent fires. Results were evaluated by comparisons with data from other coniferous forests, including those with sandy or sandy loam soils at three temperate sites under Pinus radiata D. Don in New Zealand. The very acid (pH 4.1) litter (ca. 3 cm depth) from the Zotino site contained (to 50 cm depth of mineral soil) ca. 41% of the total C (2.8 kg m −2), 34% of the total N (86 g m −2) and about 50% of the microbial biomass and respiratory and net N-mineralizing activity. The low values in the mineral soil were, except for total N which was lowest at the Zotino site, generally similar to those in two young coastal sands under P. radiata in New Zealand. Rates of C and net N mineralization in the Zotino mineral soil declined proportionately more between 13.5°C and 5.5°C than between 25 and 13.5°C. Use of temperature relationships suggested that less than half of the CO 2–C produced from the soil profile in the field under well-watered conditions during summer would have originated from microbial respiration. In litter and in mineral soil, ratios of microbial C and N-to-total C and N, microbial C-to-microbial N, rates of CO 2–C and net mineral-N production on a total C and N basis, and metabolic quotients (qCO 2 values) were all similar to at least some of the values found for the temperate pine-forest soils. Our results strongly suggest that soil organic matter content is low at this Zotino site, not because of distinctive metabolic potentials of the microbial populations, but because of environmental constraints on ecosystem processes.

Spatial and temporal fluctuation in biomass, nitrogen mineralizing reactions and mineral nitrogen in a soil cropped to barley

We compared changes in components of the N-mineralization cascade ranging from the very specific, such as a deaminase, to the highly integrated, such as biomass in a Black Chernozemic seeded to barley (Hordeum vulgare L.) under field conditions at Edmonton. Changes in enzyme content were related to soil [Formula: see text] to determine if the microbial environment changed sufficiently to exert feedback control on N mineralizing reactions and thereby to be detected. Histidase and protease were chosen as model systems for depolymerization and deamination respectively because information exists on their control in pure culture studies, on histidine content and control of histidase in soil, and assay procedures are available for soils. We observed an inverse relationship of labile histidase activity with [Formula: see text] in soils with high [Formula: see text] content and low [Formula: see text] ratio. This relationship provides indirect evidence for [Formula: see text] control of histidase content, but emphasizes that it is only one element of a complex control mechanism. Conversely, enzyme content was not rate limiting to net N mineralization, or sensitive to common control mechanisms. Biomass-C, an integrative measure of substrate supply, potential biological activity and enzymatic activity, describes net mineral-N production better than do indices of any single step. Regular spatial variability is exhibited by [Formula: see text] (and [Formula: see text]). [Formula: see text] is a product of mineralization; a substrate for immobilization, nitrification, plant uptake, and other reactions; and may also be a regulator of activity, or synthesis, of some enzymes. It is intriguing that none of the variables that influence mineral N, such as enzyme activity, biomass, or respiration, varied spatially in a statistically identifiable manner, yet [Formula: see text] did. Key words: Nitrogen mineralization, enzyme content, biomass, protease, histidase, ammonium regulation

Influence of biogas-digester effluent on crop growth and soil biochemical properties under rotational cropping

Abstract A 6-year study investigated the suitability of effluent from an anaerobic biogas digester as a fertiliser for supporting crop growth and maintaining soil biochemical levels under rotational cropping. The soil at the experimental area was predominantly a Fluvaquentic Eutrochrept. Comparisons were made with an inorganic fertiliser and a water-only treatment, using three crops (maize, oats, and kale) grown over a 2-year rotation. Dry matter yields were not consistently influenced by any of the treatments, apparently because of high soil-nutrient reserves. Thus the effectiveness of the effluent as a fertiliser could not be assessed. However, plant nitrogen concentrations were usually higher in the effluent and fertiliser treatments than in the water-only treatment for the last 3 years of the study. Soil organic C content for each treatment remained unchanged over the 6-year period. In contrast, microbial biomass, urease activity, and net mineral-N production (0–28 days) declined significantly in all ...