Interactive effects of compost and pre-planting soil moisture on plant biomass, nutrition and formation of mycorrhizas: a context dependent response

Abstract

We aimed to investigate the combined impacts of compost addition and pre-planting soil moisture conditions, on plant-available nutrients, and subsequent impacts on the biomass, nutrition and formation of AM by two important crop species. A glasshouse study was undertaken in which wheat and tomato plants were grown in compost amended or un-amended soil that was subjected to different moisture regimes prior to planting. The availability of P was strongly influenced by compost addition, but not pre-planting moisture conditions. In contrast, mineral N pools were affected by compost addition and pre-planting soil moisture conditions in complex ways. These changes in nutrient availability affected plant biomass, nutrient uptake and formation of AM. In general, plant performance was better where pre-planting soil moisture conditions were wet or dry, and worse where they involved a wet/dry cycle, and mycorrhizal colonisation was lower where compost was added to the soil. That pre-planting moisture conditions affect the biomass of subsequent crops is an important finding, the potential implications of which are considered here.

Figure 1

Concentrations of (a) NH₄⁺-N, (b) NO₃⁻-N, and (c) plant-available (Colwell) P, in soils at the time of planting, that were amended with compost (black bars), or were un-amended (white bars), and subjected to different pre-planting soil moisture conditions (see text). Values are means ± SE (n = 10). Means followed by the same letters are not significantly different at the P < 0.05 level.

Figure 2

Biomass of (a) tomato and (b) wheat; N content of (c) tomato and (d) wheat; and P content of (e) tomato and (f) wheat, grown in soils that were amended with compost (black bars), or were un-amended (white bars), and subjected to different pre-planting soil moisture conditions (see text). Values, where are means ± S.E. (n = 5), for above- and below-ground tissues are presented above- and below the x-axis, respectively. For tomatoes, above-ground biomass was for vegetative tissues only. For wheat, above-ground biomass was divided into shoots and heads, which presented as the lower and upper portions of the bars above the x-axis, respectively. Where the ANOVA revealed a significant two-way interaction bars are labelled with individual letters; means followed by the same letter are not significantly different at the P < 0.05 level. Where the main effect of compost was significant this is reported in the text. Where the main effect of pre-planting watering treatment was significant, differences between watering treatments (i.e. pooled over compost treatment within a given compost watering treatment) are indicated with letters sitting immediately above a horizontal line. N.B. valid statistical comparisons cannot be made between plant species, nor tissue types.

Figure 3

Mycorrhizal colonisation (precent root length colonised) of (a) tomato and (b) wheat, grown in soils that had been amended with compost (black bars), or were un-amended (white bars), and subjected to different pre-planting soil moisture conditions (see text). Values are means ± SE (n = 5). Means followed by the same letters are not significantly different at the P < 0.05 level. N.B. valid statistical comparisons cannot be made between plant species.

Figure 4

Pre-planting soil moisture conditions. Soil moisture conditions are expressed as a percentage of soil Water Holding Capacity (WHC). Soil that was amended with compost, and un-amended controls are represented by closed and open symbols, respectively. Pre-planting moisture treatments were Wet (circle), Dry (squares) and Cycle (triangles) and described in the text. Values are means, n = 10. Standard errors were extremely small and so are not visible on the figure at this scale, and so are not included. N. B. plants were introduced into these pots on day 40, after which all pots were maintained at 75% of WHC – see text for details.