Veterinary Science Collected Works - Research Publications
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ItemCarbon (δ13C) dynamics in agroecosystems under traditional and minimum tillage systems: a review(CSIRO PUBLISHING, 2021)Following cultivation, substantial loss of soil organic matter occurs in surface soil layers. No-till is an agronomic practice to reverse or slow the loss of soil organic matter. We reviewed 95 research papers that used 13C natural abundance of soils to quantify the impact of tillage on the C dynamics of cropping systems. New C (from current cropping systems) accumulated in the surface soil under no-till, whereas the most extreme cultivation (mouldboard ploughing) mixed new C throughout the soil. There was a decline in soil C with years of cultivation. Compared with land that had been tilled, no-till generally had little impact on the accumulation on soil organic C. Tillage and residue retention caused stratification in C stocks that depended on tillage depth, with the highest C concentrations and stocks found in the surface under no-till. Shifts in the δ13C signature indicated significant exchange of ‘new’ C for the original (old) C. Tillage methods had no impact on the size and δ13C signature of the microbial biomass pool. Change in δ13C indicates that microbial biomass rapidly incorporates new carbon. The largest change in the δ13C values (Δ13C) was observed in the coarse sand fraction, whereas the smallest change occurred in the clay fraction. Comparison of conventional vs no-till showed inconsistent results on the effect of tillage on C in the different particle size fractions. Natural 13C abundance data show that no-till cropping systems do not result in increases in soil organic C in the top 0.30 m of soil.
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ItemOn inorganic N uptake by vascular plants: Can 15N tracer techniques resolve the NH4+ versus NO3- "preference" conundrum?(WILEY, 2021-07)Abstract The relative uptake by plants of the two ionic nitrogen (N) forms, ammonium (NH4+) and nitrate (NO3−), has been the subject of much interest during the past 50 years, resulting in a considerable scientific literature. The general idea is that plants have choice, resulting in preference for either one mineral N form or the other. Unfortunately, there is no specific definition of preference or agreement on how it should be measured. In this review, we critically examine the alternative techniques that have been used to measure the relative uptake of NH4+ and NO3− by plants, including those based on unlabelled sources, 15N‐enriched mineral N forms and variations in the 15N natural abundance of mineral N sources. The main difficulty with using unlabelled N is the antecedent N in plant tissue prior to the imposition of treatments. Although 15N‐enrichment overcomes this obstacle, it is nevertheless difficult to separate uptake as 15NH4+, uptake as 15NO3− and uptake as 15NO3− derived from the nitrification of 15NH4+. With 15N natural abundance, isotopic fractionation during plant uptake complicates the interpretation of data. There is increasing evidence that plants exhibit flexibility or plasticity with respect to the use of mineral N forms rather than preference. Highlights The concept of plant preference for NH4+ or NO3− forms of mineral N is examined Experiments using 15N‐enrichment and 15N natural abundance are reviewed The direct uptake of 15NO3− is confounded with the uptake of 15NO3− derived from nitrified 15NH4+ Plants exhibit plasticity rather than preference in the acquisition of ammonium and nitrate