David R. Coventry

Background: The soil environment is responsible for sustaining most terrestrial plant life, yet we know surprisingly little about the important functions carried out by diverse microbial communities in soil. Soil microbes that inhabit the channels of decaying root systems, the detritusphere, are likely to be essential for plant growth and health, as these channels are the preferred locations of new root growth. Understanding the microbial metagenome of the detritusphere, and how it responds to agricultural management such as crop rotations and soil tillage, is vital for improving global food production. Results: This study establishes an in-depth soil microbial gene catalogue based on the living-decaying rhizosphere niches in a cropping soil. The detritusphere microbiome regulates the composition and function of the rhizosphere microbiome to a greater extent than plant type: rhizosphere microbiomes of wheat and chickpea were homogenous (65–87% similarity) in the presence of decaying root (DR) systems but were heterogeneous (3–24% similarity) where DR was disrupted by tillage. When the microbiomes of the rhizosphere and the detritusphere interact in the presence of DR, there is significant degradation of plant root exudates by the rhizosphere microbiome, and genes associated with membrane transporters, carbohydrate and amino acid metabolism are enriched. Conclusions: The study describes the diversity and functional capacity of a high-quality soil microbial metagenome. The results demonstrate the contribution of the detritusphere microbiome in determining the metagenome of developing root systems. Modifications in root microbial function through soil management can ultimately govern plant health, productivity and food security.

In the river valleys of central Tibet, farming households are active in both crop and livestock production. The emphasis is mostly placed on optimizing grain production for subsistence purposes, with little emphasis on forage production to meet livestock diets. Temperatures and rainfall favor crop growth from April to October and, with crops harvested in July or August, sufficient moisture, heat, and light resources remain unused after forage harvest. Here we report data from crop‐forage intercrop experiments that evaluated the role of using vetch (Vicia sativa L.) and lucerne (Medicago sativa L.) established by either broadcast or seeding in inter‐row spaces of cereal crops. Broadcasting vetch in early July into maturing winter sown wheat (Triticum aestivum L.) allows useful amounts of vetch forage (3 Mg ha⁻¹) produced for little extra work with minimal grain yield penalty. The approach of sowing vetch in the inter‐row space of widely spaced winter wheat or barley (Hordeum vulgare L.) was a reliable way of establishing this forage crop. However, grain yield losses (P ≤ 0.05) of approximately 16, 22, and 37% were associated with the wider row spacing (66, 50, and 33% wheat) when vetch was grown in the inter‐row space. Based on land equivalent ratios (LERs) alone, the 33% cereal density was most productive (P ≤ 0.05). However, improved understanding of forage, dairy products, and grain markets, and of the household economy on typical Tibetan farms, will be required before judgments can be made about whether farmers can profit from such intercropping approaches.

Soil acidification, as influenced by N fertiliser, stubble management, and crop rotations, was investigated using soil samples from a long-term rotation trial at Tarlee, South Australia. With the effects of combination of treatment inputs (wheat–lupin, stubble retention and N-fertiliser application), the pHCa (0–10 cm depth) declined from the starting value of 6.12 to 4.50 after a 14-year period. All of the treatments caused the soil pH to decrease. The mean acidification rates for the period 1978–1992 varied from 0.5 to 2.22 kmol H+/ha.year for different treatments. Although the rainfall at Tarlee (483 mm) is not sufficient to cause regular drainage events, it is likely that downward movement of NO3– in the soil profile has caused acidification in the surface soil. Also the retention of stubble caused more acidification than where there was regular stubble burning or removal. The acidification resulted in an increase in soil exchangeable Al and Mn and extractable Al and Mn, and the decrease in soil exchangeable base cations. The values for soluble Al (extracted in CaCl2) in 1992 were 1.58 and 2.45 mg/kg (0–10 cm depth) for the wheat–bean and wheat–lupin rotations, but the percentage of Al in the ECEC was low. It is not known whether this acidity has any impact on yields of field crops at this stage. But with soluble Al and the percentage of Al in ECEC increasing, it is possible that Al toxicity may occur in the high input-output cropping soils in South Australia.