A global meta-analysis of soil exchangeable cations, pH, carbon, and nitrogen with afforestation

Background/Question/Methods

Afforestation, the conversion of non-forested lands to forest plantations, can sequester atmospheric carbon dioxide, but the rapid growth and harvesting of biomass may deplete nutrients and degrade soils if managed improperly. The goal of this study is to evaluate how afforestation affects mineral soil quality, including pH, sodium, exchangeable cations, organic carbon, and nitrogen, and to examine the magnitude of these changes regionally where afforestation rates are high. We also examine potential mechanisms to reduce the impacts of afforestation on soils and to maintain long-term productivity.

We surveyed the available literature and located 71 studies with 153 independent sites globally with afforested plantations paired with grass, shrub, or agricultural controls. Depth of sampling varied, but we restricted our analysis to the top 30cm of mineral soil. To control for potential methodological differences between studies, we analyzed response ratios for each paried site (natural logarithm of value of afforested plantation divided by value of control). Because the response ratios were typically not normally distributed or had small sample size, the effect of afforestation on soil variables was tested non-parametrically by generating 95% confidence intervals using bootstrapping (10,000 iterations with replacement).

Results/Conclusions

Across diverse plantation types (153 sites), afforestation decreased Ca, K, and Mg, increased Na, or both. Afforestation reduced soil Ca by 29% on average (p<0.05). Afforestation by Pinus decreased soil K by 23% (p<0.05). Overall, plantations increased soil Na 71% on average (p<0.05). Average pH decreased 0.3 units (p<0.05) with afforestation. Afforestation caused a 6.7% and 15% (p<0.05) decrease in soil C and N content respectively, though the effect was driven principally by Pinus plantations (15% and 20% decrease, p<0.05). Carbon to nitrogen ratios in soils under plantations were 5.7-11.6% higher (p<0.05). In regions with high rates of afforestation, cumulative losses of N, Ca, and Mg are on the order of tens of millions of metric tons. The decreases indicate that trees take up considerable amounts of nutrients from soils; harvesting this biomass repeatedly could impair long-term soil fertility and productivity. Based on this study and a review of other literature, we suggest that proper site preparation and sustainable harvest practices, such as avoiding the removal or burning of harvest residue, could minimize the impact of afforestation on soils. These practices could slow soil compaction, erosion, and organic matter loss, maintaining soil fertility to the greatest extent possible.