nitrogen depletion rates and wood chip mulch

**"* K* <i*@thisgardencooks.com>**

This is an interesting report from folks in Seattle regarding nitrogen immobilization by fresh wood chips...

http://www.ser.org/sernw/pdf/SPU_mulch_lit_review.pdf 

\Nitrogen immobilization by fresh wood chip appears to be a real problem in some situations. While some studies have shown no growth inhibition or nitrogen immobilization from use of wood chip (Greenly and Rakow, 1995âpine chip and hardwood chip; Pickering and Shepherd, 2000âconifer bark and wood chips), a multi-year study of mulching with fresh wood chip from ground-up shipping pallets documented soil-nitrogen immobilization, and inhibition of growth and flowering of rhododendrons and river birches compared to an unmulched control and plots mulched with yard debris compost (Lloyd, Herms, Stinner and Hoitink. 2003). Nitrogen immobilization was documented at the soil surface and at 6 inch depth, which the authors attribute to nutrient scavenging by fungal hyphae, nutrient movement in soil water, and soil fauna cultivation. The mulch was removed and replaced with fresh wood chip annually, preventing breakdown and beneficial nitrogen release from the wood that would naturally occur in aging mulch. (Hoitink and Krause, 1998, suggest that removing particles smaller than 3/8 inch through screening prevents nitrogen immobilization problems).
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