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

Abstract

Abstract. 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.Across diverse plantation types (153 sites) to a depth of 30 cm of mineral soil, we observed significant decreases in nutrient cations (Ca, K, Mg), increases in sodium (Na), or both with afforestation. Across the data set, afforestation reduced soil concentrations of the macronutrient Ca by 29% on average (P , 0.05). Afforestation by Pinus alone decreased soil K by 23% (P , 0.05). Overall, plantations of all genera also led to a mean 71% increase of soil Na (P , 0.05). Mean 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 several regions with high rates of afforestation, cumulative losses of N, Ca, and Mg are likely in the range 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 in some locations. 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 sustainable practices would in turn slow soil compaction, erosion, and organic matter loss, maintaining soil fertility to the greatest extent possible.

Geographic bias of field observations of soil carbon stocks with tropical land-use changes precludes spatial extrapolation

“…Thus, our analysis assumes that soil C stocks have reached equilibrium values under current land uses. Undoubtedly, other factors not incorporated in our analyses also affect the direction and magnitude of changes in soil C stocks, including site preparation, fertilization and improved management (26), species effects (27), and legacy effects of multiple landuse transitions. However, we did not stratify according to these factors, as their effect is less studied and no georeferenced databases of these factors exist that might be used to improve predictions of soil C stock changes following land-use change.…”

“…Published studies located between 28°3 5′ N and 28°15′ S latitude were identified from previous meta-analyses and reviews (14,27,(32)(33)(34) or from searching online scientific databases. The majority of the studies were conducted between 23°N and 23°S latitudes and only a few are considered subtropical.…”

“…(14,27), we used nonparametric resampling methods to generate bias-corrected bootstrapped approximate 95% confidence intervals (CI) from 10,000 randomizations in META-WIN (37), and response effects were not weighted by sample size. Observed effect sizes were considered statistically different from zero if the 95% CI did not include zero, and land-cover transitions or other categorical grouping factors were considered different from one another if their 95% CI did not overlap.…”

Vegetation Type and Age Drive Changes in Soil Properties, Nitrogen, and Carbon Sequestration in Urban Parks under Cold Climate

“…The mechanism by which woody vegetation boosts C accumulation in soils is poorly known. Berthrong et al (2009) suggested that urban woody vegetation increases the input of organic matter with a high C/N ratio and, since litter with high C/N decomposes more slowly (Stevenson, 1994), the soils will accumulate C stocks over time. Although woody vegetation, compared to lawn, may increase the C/N ratio in urban greenspace soils (Livesley et al, 2016), other urban studies have found no evidence of such an effect (Golubiewski, 2006;Raciti et al, 2011;Edmondson et al, 2014b).…”

“…The relative proportion of C and N in the soil can be used as an indicator of soil quality, describing the relative proportion of nitrogen that is either mineralized by soil microbes or immobilized into the microbial biomass (Berthrong et al, 2009). The equal C/N ratio across all studied park soils implies that the availability of N, i.e., soil fertility (Brady and Weil, 2008), is similar beneath the three vegetation types in our study (per age class), despite the 30% higher N concentration under the evergreens than under lawns.…”