Temporal changes in SOM, N, P, K, and their stoichiometric ratios during reforestation in China and interactions with soil depths: Importance of deep-layer soil and management implications

• Larch afforestation induced SOM accumulation in surface but depletion in deep soil.
• Total N, P, their extractable forms peaked at 20–40-yr, but depleted in >40-yr forest.
• Nearly 50% of SOM and N was in 0–20 cm soil, but >60% of P and K was in deep soils.
• >30% Increases in C:N, C:P, K:P and K:extractable K were observed in >40-yr forest.

Rapid increases in the global area of plantation forests necessitate a fuller understanding of their effects on soil fertility for continuous management and nutrient supply. Stands of a forest type widely distributed in the northern hemisphere, larch (Larix gmelinii) plantations, of 3 age groups (<20 yr, 56 plots; 20–40 yr, 73 plots; >40 yr, 30 plots) were selected for the sampling of soil in 4 layers to 80 cm depth, and a total of 30 soil parameters (the concentration, storage, and vertical distribution of SOM; N; extractable N; P; extractable P; K; extractable K; and their stoichiometric ratios) were determined, and multivariate analysis of variance (MANOVA) with soil depth and age group as fixed factors was used to highlight the importance of deep-layer soil in reforestations. Nearly half of SOM, total N, extractable N were in surface 20 cm soil, while there were 60–80% of total P, extractable P, total K and extractable K in deep-layer soils (20–80 cm), indicating deep soil importance for P and K supply. The development of larch plantation forest could markedly affect 19 out of the 30 soil parameters (8 parameters showed different temporal changes between surface and deep-layer soils (P < 0.05) and the other 11 parameters were similar at different soil layers (P > 0.05)). SOM at the surface layer (0–20 cm) in >40-yr age-group was 36% higher than <20-yr age group, while in 60–80 cm soil, decreases of 35–37% were observed, these resulted in a much moderate increase in SOM concentration on overall 80 cm soil profile; Concentration and storage of total N, total P and their extractable forms experienced a mid-age (20–40-yr age group) peak when considering the overall 80 cm soil column, and the lowest values were in >40-yr age group, and their depletion was respectively 20–25%, 29–30%, 33–37% and 79–86%. All these resulted in an uplifting tendency of SOM, total N, and total P with tree age (P < 0.05) except total K (no changes). Although the larch is typically a shallow-rooted tree species, our findings indicate that soil deeper than that penetrated by most roots is still sensitive to larch growth, and the most likely age group for marked fertility changes was in the >40-yr age group. These fertility changes and the time-period for nutrient depletion are crucial for intensive management (fertilization) and budget estimation, and more attention should be paid to deep-layer soils.