Effect of microbial activity on penetrometer resistance and elastic modulus of soil at different temperatures

We explore the effect of microbial activity stimulated by root exudates on the penetrometer resistance of soil and its elastic modulus. This is important because it is a measure of the mechanical strength of soil and it correlates closely with the rate of elongation of roots. A sandy soil was incubated with a synthetic root exudate at different temperatures, for different lengths of time and with selective suppression of either fungi or bacteria. The shape of the temperature response of penetrometer resistance in soil incubated with synthetic exudate was typical of a poikilothermic temperature response. Both penetrometer resistance and small strain shear modulus had maximum values between 25 and 30°C. At temperatures of 20°C and less, there was little effect of incubation with synthetic root exudate on the small strain shear modulus, although penetrometer resistance did increase with temperature over this range (4–20°C). This suggests that in this temperature range the increase in penetrometer resistance was related to a greater resistance to plastic deformation. At higher temperatures (> 25°C) penetrometer resistance decreased. Analysis of the DNA sequence data showed that at 25°C the number of Streptomyces (Gram‐positive bacteria) increased, but selective suppression of either fungi or bacteria suggested that fungi have the greater role with respect to penetrometer resistance

Incidental phosphorus and nitrogen loss from grassland plots receiving chemically amended dairy cattle slurry

peer-reviewedChemical amendment of dairy cattle slurry has been shown to effectively reduce incidental phosphorus (P) losses in runoff; however, the effects of amendments on incidental nitrogen (N) losses are not as well documented. This study examined P and N losses in runoff during three simulated rainfall events 2, 10 and 28 days after a single application of unamended/chemically amended dairy cattle slurry. Twenty-five hydraulically isolated plots, each measuring 0.9 m by 0.4 m and instrumented with runoff collection channels, were randomly assigned the following treatments: (i) grass-only, (ii) slurry-only (the study-control), (iii) slurry amended with industrial grade liquid alum comprising 8% Al2O3, (iv) slurry amended with industrial grade liquid poly-aluminum chloride (PAC) comprising 10% Al2O3, and (v) slurry amended with lime. During the first rainfall event, lime was ineffective but alum and PAC effectively reduced dissolved reactive P (DRP) (by 95 and 98%, respectively) and total P (TP) flow-weighted-mean-concentrations (by 82 and 93%, respectively) in runoff compared to the study-control. However, flow-weighted-mean-concentrations of ammonium–N (NH4–N) in runoff were increased with alum- (81%) and lime-treated (11%) slurry compared to the study-control whereas PAC reduced the NH4–N by 82%. Amendments were not observed to have a significant effect on NO3–N losses during this study. Slurry amendments reduced P losses for the duration of the study, whereas the effect of amendments on N losses was not significant following the first event. Antecedent volumetric water content of the soil or slope of the plots did not appear to affect runoff volume. However, runoff volumes (and consequently loads of P and N) were observed to increase for the chemically amended plots compared to the control and soil-only plots. This work highlights the importance of considering both P and N losses when implementing a specific nutrient mitigation measure.Teagasc Walsh Fellowship Schem