Soil organic N dynamics and stand quality in Pinus radiata pinewoods of the temperate humid region

Soil organic-N dynamics, its controlling factors and its relationships with stand quality were studied in the 0-15 cm soil layer of 24 pinewoods with contrasting age, productivity and parent material (granite; acid schists), searching for N variables useful to predict stand growth and site quality. No significant differences were found between young and old stands for any of the N variables considered, nor two- or three-order interactions among stand age, site quality and parent material. The soil total-N content, which was correlated positively with the Al oxides content (a soil organic matter (SOM) stabilizing agent), did not vary significantly according to parent material, but it was lower (P≤0.02) in stands with high than with low site index (2.68±1.11 and 3.97±1.13 g N kg⁻¹ soil, respectively). The soil δ¹⁵N ranged from +3.5 to +6.5 δ, without significant differences among stand groups, and it was negatively correlated with water holding capacity, exchangeable bases, Al oxides and N content, suggesting that: (i) N losses by NO₃⁻ leaching are the most important controlling factor of δ¹⁵N in these temperate humid region soils; and (ii) soil N richness is related with limited N losses, which discriminate against ¹⁵N. At any incubation time, no significant differences were found in soil inorganic-N content among stand groups (7.78±4.57, 39.33±16.20 and 67.80±26.50 mg N kg⁻¹ soil at 0, 42 and 84 d, respectively). During the incubation, the relative importance of ammonification decreased and that of the nitrification increased. The net N mineralization rate (NNMR, in percentage of organic N) was significantly higher in granite than in schists soils at both 42 d (1.24±0.34 and 0.75±0.37%, respectively) and 84 d (2.18±0.56 and 1.53±0.66%, respectively). In high quality pinewoods, the NNMR at 42 and 84 d (1.16±0.45 and 2.12±0.79%, respectively) were significantly higher than in low quality stands (0.83±0.35 and 1.59±0.45%, respectively). This result, together with those on soil total-N and inorganic-N supply, suggests that soil N dynamics in low and high quality stands are different: in the former there is a bigger N pool with a slower turnover, whereas in the latter there is a smaller N pool with a faster turnover, both factors being nearly compensated, making the soil available N supply in both types of stand similar. After 42 and 84 d of incubation, the NNMR and the nitrification rates were higher in the coarse textured soils, likely due to the low physical and chemical protection of their SOM; both rates were positively correlated with available P, exchangeable K⁺ and CEC base saturation, suggesting strong relationships among the availabilities of the main plant nutrients, and they increased with SOM quality (low C-to-N ratio). The strong negative correlation of site index with soil total-N (r=−0.707; P≤0.005), and its positive correlations with NNMR after 42 and 84 d of incubation, suggested that site quality and potential productivity are closely related to soil organic-N dynamics. Half of the site index variation in the stands studied could be predicted with a cheap and easy analysis of soil N content, the prediction being slightly improved if soil δ¹⁵N is included and, more significantly, by including N mineralization measurements.