No abstract is provided for this article.
Plastic film mulch (PFM) controls weeds and increases yields, making it attractive to vegetable growers; biodegradable PFMs potentially reduce the harms associated with conventional PFMs. PFMs increase soil biological activity, accelerating the decomposition of soil organic matter and potentially increasing emissions of some greenhouse gases (GHGs). Conversely, they are a barrier to rainfall infiltration and gas exchange, reducing harmful nitrate (NO 3 - ) leaching and ammonia (NH 3 ) volatilisation. The effects of PFMs on the processes resulting in GHG emissions are not well explored outside conventionally grown commodity crops in major growing regions. To address this, we conducted a field experiment on an organic vegetable farm with a temperate maritime climate. We measured nitrous oxide (N 2 O), methane (CH 4 ), carbon dioxide (CO 2 ) and potential NH 3 emission from the soil, growing leeks or cabbages, with or without biodegradable PFM and amended with poultry manure or green-waste compost. Averaged across both crops, yield was 26% higher with PFM; potential NH 3 emissions were 18% lower (43% on a yield-scaled basis) in mulched treatments than unmulched; CH 4 emissions were not significantly affected. Yield-scaled N 2 O emissions were 62% higher in mulched leeks than unmulched but 56% lower in mulched cabbages than unmulched; this coincided with higher soil NO 3 - content in mulched leeks than either unmulched crop or mulched cabbages. Results were not obtained for CO 2 , so partial global warming potential (GWP) and greenhouse gas intensity (GHGI) were determined mainly by N 2 O emissions. Overall, our results indicate that biodegradable PFM can potentially reduce harmful gaseous N emissions in organic horticulture.
In crop plants, aluminum (Al) rhizotoxicity is a major problem worldwide; however, the cause of Al toxicity remains elusive. The effects of Al on the inositol 1,4,5-trisphosphate (Ins[1,4,5]P3)-mediated signal transduction pathway were investigated in wheat roots. Exogenously applied Al (50 [mu]M) rapidly inhibited root growth (<2 hr) but did not affect general root metabolism. An Ins(1,4,5)P3 transient was generated in root tips, either before or after exposure to Al for 1 hr, by treating the roots with H2O2 (10 mM). Background (unstimulated) levels of Ins(1,4,5)P3 were similar in both Al-treated and Al-untreated root apices. However, H2O2-stimulated levels of Ins(1,4,5)P3 in root apices showed a significant (>50%) reduction after Al exposure in comparison with untreated controls, indicating that Al may be interfering with the phosphoinositide signaling pathway. When phospholipase C (PLC) was assayed directly in the presence of Al or other metal cations in microsomal membranes, AlCl3 and Al-citrate specifically inhibited PLC action in a dose-dependent manner and at physiologically relevant Al levels. Al exposure had no effect on inositol trisphosphate dephosphorylation or on a range of enzymes isolated from wheat roots, suggesting that Al exposure may specifically target PLC. Possible mechanisms of PLC inhibition by Al and the role of Ins(1,4,5)P3 in Al toxicity and growth are discussed. This study provides compelling evidence that the phytotoxic metal cation Al has an intracellular target site that may be integrally involved in root growth.
In natural plant–soil systems and following incorporation into plants in agricultural systems, most nitrogen enters soil as protein or other biological polymers. Before this nitrogen can be taken up by soil microorganisms or plants, it must be cleaved to a form which can be transported into the organism. It is well known that soil microorganisms and many plants can utilise l-amino acids and a growing body of evidence shows that short l-peptides can be taken up without further modification. However, there has been very little empirical investigation of rates of uptake in soil. d-amino acids and their peptides are less abundant in soil than their l-enantiomeric counterparts, but do occur in significant quantities. To date their potential rate of cycling in the soil solution remains virtually unknown. We directly measured rates of uptake of 14C-labelled l- and d-alanine and their di- and tripeptides in a temperate agricultural soil. All were taken up extremely rapidly by soil microorganisms. Half-times for l-enantiomers and the d-amino acid monomer were all less than 1 min, with l-peptides accounting for the highest potential fluxes (9 nmol N g−1 DW soil min−1), more than double that of the amino acid monomer. Uptake of d-peptides was slowest with half-times in soil solution of 10 and 36 min (di- and tripeptide, respectively). Availability of l-peptides had a much stronger effect on the rate of uptake of the amino acid monomer than the amino acid monomer had on the uptake of the peptides, suggesting a microbial preference for peptides. We suggest that very high potential N fluxes and an apparent microbial preference for l-peptides indicates that most N is cycled through the soil solution as l-peptides early in protein degradation, and that this is the site of fiercest competition for soil N.
The addition of organic matter to soil is frequently viewed as a vital intervention to maintain soil quality. The aim of this study was to investigate the temporal response of the soil macroorganic fraction to different organic coffee farming practices (e.g., plant residue, earthworm and microbial inocula addition). Three density fractions of macroorganic matter (>150 μm) were studied during 1 year after adding shade tree (Erythrina poeppigiana) pruning residues to the soil (5 t ha−1 twice at 6 monthly intervals). Soil macroorganic matter represented only a small proportion of total soil organic matter (SOM) (3–6% of total). Even though the total amount of SOM did not change over time, significant temporal changes in the size of the macroorganic fraction were observed that appeared to be largely independent of the management regime. The light density fraction seemed to be the most responsive fraction and this study suggests that it may provide a qualitative indicator of the ‘active’ fraction of SOM; the size of the macroorganic fraction did not provide a reliable indicator of the rate of litter decomposition or nutrient release. The addition of microbial inoculants and earthworms had only a small and inconsistent effect on macroorganic matter dynamics and these practices appeared to offer little agronomic benefit. This study highlights the need for continued organic matter inputs to maintain soil C reserves and preserve soil organic quality in tropical organic farming systems.
