Effects of exogenous nitrogen import on variations of nutrient in decomposing litters of Suaeda salsa in coastal marsh of the Yellow River estuary, China

To explore the effects of exogenous nitrogen (N) import on decomposition of Suaeda salsa in coastal marsh of the Yellow River estuary, the decomposition rates and the variations of nutrient (C, N, P, and S) in decomposing litters were investigated from May 2014 to November 2015. The in situ experiment included four N addition treatments: N0 (no N import treatment, 0 g N·m⁻²·year⁻¹), Nlow (low N import treatment, 3.0 g N·m⁻²·year⁻¹), Nmedium (medium N import treatment, 6.0 g N·m⁻²·year⁻¹), and Nhigh (high N import treatment, 12.0 g N·m⁻²·year⁻¹). Results showed that N enrichment showed significant influence on the decomposition rate of S. salsa, in the order of Nmedium (0.00112 d⁻¹) > Nhigh (0.00096 d⁻¹) > Nlow(0.00092 d⁻¹) > N0 (0.00075 d⁻¹) (p < 0.05). Compared to the N0 treatment, the decomposition rates of S. salsa in the Nlow, Nmedium, and Nhigh treatments increased by 22.76%, 49.33%, and 28.00%, respectively. The contents of TC in decomposing litters in the four N import treatments generally showed a decreasing trend, while those of TN and TP showed an increasing trend. By comparison, dissimilar variations of TS contents in decomposing litters were observed among the four treatments. The differences in decomposition rate and nutrient variation among the four N addition treatments might be dependent on the alterations of substrate quality in decomposing litters. With a few exceptions, stocks of C and S in decomposing litters generally evidenced the export from litters to the environment, while those of N and P showed net accumulation. As N addition reached Nmedium level, although the C released from decomposing litters to the surroundings was stimulated, the S return was restrained. Moreover, N additions generally promoted the accumulation of N and P in decomposing litters. This paper found that, with increasing N addition, the decomposition rates and the nutrient variations in S. salsa would be altered greatly and the alteration was particularly evident in the Nmedium treatment. From the perspective of nutrient return, as N enrichment reached or exceeded Nmedium level in future, the deficient status of P and S in S. salsa marsh might be serious, which would affect the stability and health of ecosystems.