Initial shifts in nitrogen impact on ecosystem carbon fluxes in an alpine meadow: patterns and causes

Bing Song, Jian Sun, Qingping Zhou, Ning Zong, Linghao Li, Shuli Niu
2017 Biogeosciences  
<p><strong>Abstract.</strong> Increases in nitrogen (N) deposition can greatly stimulate ecosystem net carbon (C) sequestration through positive N-induced effects on plant productivity. However, how net ecosystem CO<sub>2</sub> exchange (NEE) and its components respond to different N addition rates remains unclear. Using an N addition gradient experiment (six levels: 0, 2, 4, 8, 16, 32<span class="thinspace"></span>gN<span class="thinspace"></span>m<sup>−2</sup><span
more » ... sup>−1</sup>) in an alpine meadow on the Qinghai–Tibetan Plateau, we explored the responses of different ecosystem C fluxes to an N addition gradient and revealed mechanisms underlying the dynamic responses. Results showed that NEE, ecosystem respiration (ER), and gross ecosystem production (GEP) all increased linearly with N addition rates in the first year of treatment but shifted to N saturation responses in the second year with the highest NEE (−7.77<span class="thinspace"></span>±<span class="thinspace"></span>0.48<span class="thinspace"></span>µmol<span class="thinspace"></span>m<sup>−2</sup><span class="thinspace"></span>s<sup>−1</sup>) occurring under an N addition rate of 8<span class="thinspace"></span>gN<span class="thinspace"></span>m<sup>−2</sup><span class="thinspace"></span>yr<sup>−1</sup>. The saturation responses of NEE and GEP were caused by N-induced accumulation of standing litter, which limited light availability for plant growth under high N addition. The saturation response of ER was mainly due to an N-induced saturation response of aboveground plant respiration and decreasing soil microbial respiration along the N addition gradient, while decreases in soil microbial respiration under high N addition were caused by N-induced reductions in soil pH. We also found that various components of ER, including aboveground plant respiration, soil respiration, root respiration, and microbial respiration, responded differentially to the N addition gradient. These results reveal temporal dynamics of N impacts and the rapid shift in ecosystem C fluxes from N limitation to N saturation. Our findings bring evidence of short-term initial shifts in responses of ecosystem C fluxes to increases in N deposition, which should be considered when predicting long-term changes in ecosystem net C sequestration.</p>
doi:10.5194/bg-14-3947-2017 fatcat:3kf65fj4pze7bmzenxof4rmcfa