Carbon, nitrogen and phosphorus net mineralization in organic horizons of temperate forests: stoichiometry and relations to organic matter quality

The rates of mineralization processes influence C sequestration and soil fertility, but despite their importance for ecosystem functioning, C, N and P net mineralization rates are seldom investigated together. Hence, we studied the relationships between net mineralization rates and organic matter stoichiometry in an 8-week incubation experiment with Oi, Oe and Oa horizon material of six beech, one spruce and one pine site. We determined C, N and P net mineralization rates, organic C quality and C: N: P stoichiometry. Net N mineralization only occurred below molar organic matter C: N ratios of 40 (Oi) or 28 (Oa) and N: P ratios of 42 (Oi) or 60 (Oa), and increased with decreasing C: N and N: P ratios. Net P mineralization only occurred below C: P ratios of 1400 (Oi) and N: P ratios of 40 (Oi), and increased with decreasing C: P and N: P ratios. Net N and P mineralization were strongly positively correlated with each other (r = 0.64, p < 0.001), whereas correlations of both net N and net P mineralization with C mineralization were weak. The average C: N: P stoichiometry of net mineralization was 620: 4: 1 (beech, Oi), 15,350: 5: 1 (coniferous, Oi), 1520: 8: 1 (Oe) and 2160: 36: 1 (Oa). On average, ratios of C: N net mineralization were higher, and ratios of N: P net mineralization lower than organic matter C: N and N: P ratios. This difference contributed to the decrease of C: N ratios and increase of N: P ratios from the Oi to the Oa horizons. In conclusion, the study shows that C, N and P net mineralization rates were closely correlated with the organic matter stoichiometry and that these correlations were modified by the degree of decomposition of the organic matter.