Decomposition nitrogen is better retained than simulated deposition from mineral amendments in a temperate forest

Abstract

Nitrogen (N) deposition (N DEP ) drives forest carbon (C) sequestration but the size of this effect is still uncertain. In the field, an estimate of these effects can be obtained by applying mineral N fertilizers over the soil or forest canopy. A 15 N label in the fertilizer can be then used to trace the movement of the added N into ecosystem pools and deduce a C effect. However, N recycling via litter decomposition provides most of the nutrition for trees, even under heavy N DEP inputs. If this recycled litter nitrogen is retained in ecosystem pools differently to added mineral N, then estimates of the effects of N DEP on the relative change in C (ΔC/ΔN) based on short-term isotope-labelled mineral fertilizer additions should be questioned. We used 15 N labelled litter to track decomposed N in the soil system (litter, soils, microbes, and roots) over 18 months in a Sitka spruce plantation and directly compared the fate of this 15 N to an equivalent amount in simulated N DEP treatments. By the end of the experiment, three times as much 15 N was retained in the O and A soil layers when N was derived from litter decomposition than from mineral N additions (60% and 20%, respectively), primarily because of increased recovery in the O layer. Roots expressed slightly more 15 N tracer from litter decomposition than from simulated mineral N DEP (7.5% and 4.5%) and compared to soil recovery, expressed proportionally more 15 N in the A layer than the O layer, potentially indicating uptake of organic N from decomposition. These results suggest effects of N DEP on forest ΔC/ΔN may not be apparent from mineral 15 N tracer experiments alone. Given the importance of N recycling, an important but underestimated effect of N DEP is its influence on the rate of N release from litter.

Litter Management as a Key Factor Relieves Soil Respiration Decay in an Urban-Adjacent Camphor Forest under a Short-Term Nitrogen Increment

“…In our N-saturated system, litter manipulation had an immediate effect on soil N pool, such as direct positive effects on (NH 4 -N) and (NO 3 -N) concentration ( Table 2, Figures 3 and 6). Similar to previous studies, the results suggested that litter decomposition increased the (NH 4 -N) pool [55], being retained in the soil litter layer, partitioned differently than mineral N fertilizer 'deposition' [56,57]. Furthermore, (NO 3 -N) of CK and LA in N3 plots showed the significantly higher mean values than other treatments (Figure 3b), which suggests that litter, to some extent, could mitigate the (NO 3 -N) leaching [58].…”