Spring 2012 - Buckeye District
Figure 1. Conceptual model illustrating the effect of organic mulches on plant nitrogen (N) and carbon (C) uptake, and C allocation. White circles represent main inputs and transitional forms of C in the mode. Black squares represent processes associated with soil N availability and soil environmental conditions. White squares represent plant processes associated with resource acquisition. Gray squares represent plant processes associated with photosynthetic allocation (Lloyd, 2009). As the decomposition process completes, the 18 microorganisms release the inorganic Nitrogen back into the soil for use by the plant. In fact some research suggests that only the surface of the soil is affected by the initial Nitrogen defi ciency and that low N condition may actually inhibit the germination and growth of seeds. This would indicate that the initial N depletion could be avoided by simply adding a dose of N at the time of mulch application that will bring the C:N ration between 35:1 and 20:1.
Soil conditions can also infl uence decomposition rates and consequently soil N availability. Microbial biomass and activity are sensitive to changes of soil pH, temperature, moisture and aeration. However, the responses to change in these variables fl uctuate among decomposer community. For example, fungi are able to develop and reproduce in a pH range of 3.5 to 8.5 while most bacteria cannot survive at a pH lower than 6.5 (Davet, 2004). Since the decomposition of mulch tends to be an alkaline reaction, some soil amendment such as sulfur maybe necessary in soils already Table 1: Decomposition rate of various amendments. 19 above a pH of 7.0. It is interesting that bacteria growth is inhibited at the lower pH range that is preferred by roses. The type of mulch used has a great bearing on the rate of decomposition as shown in the following table. Comparatively the lower the Nitrogen in the mixture the longer the process will take but other factors affect the rate as well. Water content is a signifi cant factor and as shown in Table 2, mulch material has a wide range of water permeability and retention. Amendment Decomposition rate Grass clippings, manures Rapid decomposition (days to weeks) Composts Moderate decomposition (about six months) Wood chips (redwood, cedar), hardwood bark, peat Slow decomposition (possibly years) (J.G. Davis, 2000) Table 2: Permeability and water retention of various soil amendments. Amendment Permeability Water Retention Fibrous Peat Wood chips Hardwood bark (J.G. Davis, 2000) low-medium high high very high low-medium low-medium