Ornamental Plants
Annual Reports and Research Reviews
2001

Special Circular 186-02

Mulch Effects on Soil Microbial Activity, Nutrient Cycling, and Plant Growth in Ornamental Landscapes

Mulch and Nutrient Cycling: Research Results

Mulching with composted yard waste and ground wood pallets had dramatic effects on soil organic matter, microbial activity, and nitrogen cycling that were apparent after only one season. Both mulches increased organic matter content of the soil relative to the bare soil control, with the yard waste mulch having the most substantial effect (Figure 2). Both mulches also increased microbial biomass as indicated by increased microbial nitrogen and a doubling of soil respiration (Figure 3). These results are consistent with the hypothesis that soil microbes are carbon limited, and that the addition of organic carbon can increase microbial biomass in the soil.

Figure 2. Effect of mulching with composted yard waste and recycled wood pallets on soil organic matter. Means (one standard error) with different letters are significantly different (LSD test, p <0.05).

The effects that the increased organic matter and microbial activity had on nitrogen availability and plant growth, however, were highly dependent on the C:N ratio of the mulch. The low C:N composted yard waste mulch dramatically increased total extractable soil nitrogen, while mulching with ground wood had little effect (Figure 4a). This is not surprising given the high concentration of nitrogen in the yard waste mulch (about 2%) relative to that of the wood pallet mulch (less than 0.5%).

Most of the total soil nitrogen pool was tied up by soil microbes in all treatments, but the proportion immobilized by microbes was higher in plots mulched with wood pallets (83%) than in the bare soil (76%) or composted yard waste (72%) treatments (Figure 4b). Microbial immobilization of such a high proportion of the already small pool of total nitrogen in the wood pallet plots would leave little left for plants. In the yard waste treatment, on the other hand, immobilization of a smaller proportion of a much larger nitrogen pool should result in much higher levels of nitrogen available for plants.

Indeed, the rate at which nitrogen was released from decomposing organic matter (nitrogen mineralization rate) was much higher in plots mulched with composted yard waste than in the wood pallet treatment (Figure 4c). This greatly increased nitrogen in forms available to plants (dissolved organic N, ammonium, and nitrate) (Figure 4d), and ultimately nutrient uptake (foliar nitrogen concentrations were 20 to 25% higher in the composted yard waste than in the bare soil or wood pallet treatments), as well as growth of river birch and rhododendron (Figure 5). Furthermore, mulching with composted yard waste increased flower production of rhododendron by more than 300% relative to the wood pallet and bare soil treatments (Figure 6).

Mulching with yard waste also had other substantial beneficial effects. Available phosphorus and potassium were increased, as was soil cation exchange capacity. Furthermore, bulk density was decreased by 10%, which improves soil tilth and reduces compaction. Fertilization had no additional effect on the growth of plants mulched with composted yard waste (Figure 5), indicating that nutrients released by decomposition of the compost were able to meet fully the requirements of both microbes and plants, making additional fertilization unnecessary.

Figure 3. Effect of mulching with composted yard waste and recycled wood pallets on microbial biomass as indicated by the amount of nitrogen contained in soil microbes (A) and rate of microbial respiration (B). Means (one standard error) with different letters are significantly different (LSD test, p <0.05).

Fertilization of plants growing in bare soil increased their growth to the same level as those mulched with composted yard waste, further indicating that mulching with this compost can serve as high-quality organic fertilizer.

On the other hand, the very high degree of microbial immobilization of nitrogen in the wood pallet treatment greatly reduced the rate of nitrogen mineralization (Figure 4c), which resulted in a much smaller pool of nitrogen in forms available for plant uptake (Figure 4d). A similar pattern was observed for soil phosphorus levels. Not surprisingly, nutrient uptake was reduced and plants grew much slower when mulched with recycled wood pallets (Figure 5). These results are consistent with the hypothesis that soil microbes are better competitors for nutrients than are plants, and that addition of organic matter with high C:N ratios can induce nutrient deficiencies in plants by stimulating microbial growth.

Figure 4. Effect of mulching with composted yard waste and recycled wood pallets on total extractable soil nitrogen (A), percent of total soil nitrogen immobilized by soil microbes (B), rate of nitrogen mineralization (C), and amount of nitrogen in forms available to plants (NH4, NO3, NO2, and dissolved organic nitrogen) (D). Means (one standard error) with different letters are significantly different (LSD test, p <0.05).

Fertilization relaxed the competition between plants and microbes for nitrogen and phosphorus in the ground wood treatment, thereby increasing plant growth. However, fertilization increased growth of rhododendron only to the level of plants growing in the untreated bare soil (Figure 5a), indicating that fertilization was able to compensate only partially for the nitrogen immobilizing effects of the wood pallet mulch. Fertilization of rhododendrons in the bare soil treatment resulted in plants that were 100% larger than fertilized plants that were mulched with ground wood pallets. However, fertilization increased the growth of river birch mulched with wood pallets to levels nearly equal to that of fertilized plants in the bare soil treatment (Figure 5b). This indicates that river birch was better able to compete with microbes for nitrogen than was Pioneer Silvery Pink rhododendron. Surprisingly, fertilization had little effect on soil organic matter, microbial biomass, or total extractable nitrogen. Fertilization did increase soil nitrate levels, but the effect was small and short-lived relative to the dramatic effects of mulching with composted yard waste.

Figure 5. Effect of fertilization and mulching with composted yard waste and recycled wood pallets on total above-ground biomass of Pioneer Silvery Pink rhododendron (A), and radial trunk growth of Heritage river birch (B). Means (one standard error) with different letters are significantly different (LSD test, p <0.05).

Increased plant growth in response to mulching has been attributed primarily to conservation of soil moisture and weed suppression. In our study, neither of the mulches had any effect on soil moisture or average soil temperature, and plots were fastidiously weeded, so these variables were not a factor. Rather, the primary effects of mulches were conclusively linked to the impacts of their C:N ratio on microbial biomass and nutrient cycling as they decomposed. Clearly, understanding the dominating influence of soil microbes on nitrogen availability is key to understanding the dynamics of soil fertility.

Figure 6. Effect of mulching with composted yard waste and recycled wood pallets on flower production of Pioneer Silvery Pink rhododendron. Means (one standard error) with different letters are significantly different (LSD test, p <0.05).