Intact Microcosms Research Articles

Ex situ soil respiration assessment using minimally disturbed microcosms and dried–sieved soils; comparison of methods to assess soil health

Soil respiration measurements are commonly used as soil health indicators. Several ex situ soil respiration methods exist, but comparative performances between them have rarely been analyzed. Specifically, there is a lack of comparisons between intact microcosms and destructive methods. The objective of this study was to analyze and compare three different ex situ soil respiration methodologies: minimally disturbed microcosms using fresh soil, dried–sieved 24 h burst test, and dried–sieved 10-day incubation. We hypothesized that ( i) the respiration rates for the three methods are correlated to each other; ( ii) the respiration rates are strongly correlated with soil physico-chemical parameters; ( iii) disturbance caused by drying and sieving reduces regression coefficients compared with microcosms; and ( iv) drying and sieving soil produces larger respiration rates. Soil was collected in the Province of New Brunswick, Canada. Total carbon and nitrogen (C:N), pH, aggregate stability, total dissolved C and N, NO3 and NH4, texture, and labile C were determined prior to incubations. Our results showed that the three methods had CO2 efflux in similar ranges. However, all the methods had low to no significant correlations between soil physico-chemical parameters and respiration. Total dissolved N had the strongest correlation with CO2 efflux. The results of the microcosm method significantly correlated with the results for 24 h burst test but not with the 10-day incubation method. We conclude that drying and sieving soil prior to performing ex situ soil heterotrophic respiration measurements using the 24 h burst tests can produce cautiously reliable results. Despite the disturbance, results from the 24 h burst tests are comparable with the results of the microcosm method.

How much does NH-N contribute to mineral-N losses in N-impacted acid soils under grassland in the UK? A microcosm study

It is hypothesised that NH-N is more mobile in soils than often thought. However, because litter layers and live vegetation can modify N species transformation in soils, intact microcosms were employed to investigate NH-N, NO-N and total dissolved inorganic N (DIN) leaching in soils from across an unfertilised and N-impacted grassland in York, UK. Simulated rain of diverse compositions was applied to five intact core microcosms, and leachate samples were collected at regular intervals. Leaching with simple simulated rain, consisting of appropriately dilute NaCl solution, showed that broadly comparable NH-N and NO-N leaching would occur from freely draining profiles. Profiles of heavy textured soils result in marginal DIN losses, primarily as NH-N because of denitrification, but possibly substantial losses as NH-N via overland flow during heavy precipitation. Further sequential leaching experiments on freely draining profiles with rain containing supplementary NH-N showed enhanced NH-N and NO-N leaching in two of three pairs of microcosms. However, one profile showed consistently lower NH-N leaching associated with the development of a consistently higher nitrification rate. The marked difference between profiles was thought to be due to the presence of a more distinct surface litter layer in one profile, compared with observable litter incorporation into subsoils for other profiles. Effects on mineral N species mobilities of drying and rewetting were also assessed. This confirmed that a delay of several days is required to re-establish the microbial population and its activities. Final phases, in which NO-N and then NH4NO3 were added via the rain, confirmed the mobility of N pollutant inputs through the freely draining soils.

N dynamics in the rhizosphere of Pinus ponderosa seedlings

Transformations of inorganic N were studied in microcosms containing Pinus ponderosa seedlings. We assessed the potential for immobilization and mineralization of NH 4 + in soils collected from adjacent to coarse, fine and young roots and in soil more than 5 mm from any root. Rates of mineralization and immobilization of NH 4 + in soils collected from adjacent to roots were >50% higher than those in soils more than 5 mm from any root. C input estimates suggest that soils adjacent to fine or young roots could have supported immobilization rates higher than those observed. We determined the response of mineralization and immobilization rates to increased NH 4 + in rhizosphere and bulk soil. Increasing NH 4 + addition resulted in proportional increases in immobilization rates in both rhizosphere and bulk soils but did not affect mineralization rates. The increased immobilization rates with increasing NH 4 + addition and the failure to predict immobilization rates based on C supply suggested that in the short-term, immobilization rates were limited by NH 4 + supply rather than C availability. In intact microcosms, we determined the transformation rates of inorganic N using a combined 15N pool dilution and tracer approach. The rates of mineralization, immobilization of NH 4 + and NO 3 −, nitrification and plant uptake were determined over 48 h. Rates in the root-zone were compared to rates in soils from which roots had been excluded for 2 weeks before labeling. The elimination of active roots from soil regions did not significantly change the production or the total consumption of NH 4 + in those regions. The presence of roots reduced microbial consumption of NH 4 + by nitrifiers and heterotrophs. Pine roots were successful competitors with microorganisms for limited inorganic N, but were more successful when the N source was NO 3 − vs NH 4 +. Plants accounted for 30% of the total NH 4 + consumption, but 70% of total NO 3 − consumption.