The effects of harvest and site preparation practices on long-term nutrient dynamics in forest plantations must be understood in order to ensure long-term sustainability in these intensively-managed systems. In this report, we present the results of two studies that address these effects. In the first study, we compared C and N dynamics in mixtures of litter (ceanothus and ponderosa pine) with those in an unmixed pine litter to determine whether the presence of mixed litters promotes the long-term accrual and retention of soil C and N. In the laboratory, we incubated mixed and unmixed litters in soils collected from plantations with and without understory vegetation. The presence of the N-fixing ceanothus significantly enhanced C and N mineralization, and the effect was more pronounced in the soil collected from plots with understory.

In the second part of this study, two different levels of soil compaction (SC) and forest floor removal (FFR) were used to assess their impact on tree growth, N uptake, and N status on ponderosa pine in plantations having soils of different textures. Tree growth analysis showed that in the site with loamy soil, soil compaction was beneficial to tree growth if the forest floor was retained. Compaction effect was not significant if the forest floor was removed. In clayey soil, SC depressed tree growth in the absence of forest floor. In sandy loam soil, SC positively influenced growth in the absence of forest floor. The recovery of isotopically-labeled N fertilizer in foliage of the upper crown was clearly detectable at all sites. In loamy and clayey soils, SC positively influenced recovery if the forest floor was present. No significant effects of compaction on N uptake were observed with total removal of the forest floor. In sandy loam soil, the contribution of SC to the variation of recovery was significant both with and without forest floor. Results for the loamy and clayey sites suggest that in compacted soils with retained litter, SC favored N uptake by improving unsaturated water flow and root:soil contact or by decreasing microbial activity. In sandy soils, the positive effects of compaction on N uptake may have been influenced by increased soil pore connectivity.