Understory vegetation plays the key role in sustaining soil microbial biomass and extracellular enzyme activities

Abstract

<p><strong>Abstract.</strong> While we know that understory vegetation affects the soil microbial biomass and extracellular enzyme activities in subtropical Chinese fir (<i>Cunninghamia lanceolata</i>) forests, we are less certain about the degree of its influence. We determined the degree to which the soil abiotic and biotic properties, such as PLFAs and extracellular enzyme activities, were controlled by understory vegetation. We established a paired treatment in a subtropical Chinese fir plantation, which comprised one plot from which the understory vegetation and litter were removed (None) and another from which the litter was removed but the understory vegetation was left intact (Understory). We evaluated how the understory vegetation influenced the soil abiotic properties, the bacterial, fungal, and actinobacterial PLFAs, and the activities of five hydrolases and two oxidative enzymes. The dissolved organic carbon (DOC), particulate organic carbon, soil organic carbon, ammonia nitrogen (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="8aeb386a576ed6c8280ae774099f80e4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-15-4481-2018-ie00001.svg" width="24pt" height="15pt" src="bg-15-4481-2018-ie00001.png"/></svg:svg></span></span>–N), and total nitrogen contents and soil moisture were 18<span class="thinspace"></span>%, 25<span class="thinspace"></span>%, 12<span class="thinspace"></span>%, 34<span class="thinspace"></span>%, 8<span class="thinspace"></span>%, and 4<span class="thinspace"></span>% lower in the None treatments than in the Understory treatments, respectively (<span class="inline-formula"><i>P</i>&amp;lt;0.05</span>). Soil bacterial, fungal, and total PLFAs, and the potential activities of <span class="inline-formula"><i>β</i></span>-1,4-glucosidase (<span class="inline-formula"><i>β</i></span>G), <span class="inline-formula"><i>β</i></span>-1,4-N-acetylglucosaminidase, phenol oxidase, and peroxidase, were as much as 24<span class="thinspace"></span>% lower in None treatments than in Understory treatments (<span class="inline-formula"><i>P</i>&amp;lt;0.05</span>). The specific activities of C-acquiring enzymes were as much as 41<span class="thinspace"></span>% higher (<span class="inline-formula"><i>P</i>&amp;lt;0.05</span>), and the ratio of C- to N-acquiring enzymes was also higher in the None treatments than in the Understory treatments. This suggests that in the absence of understory vegetation microbes invested more in C acquisition than N acquisition because the carbon (C) inputs were less labile. The negative relationship between DOC and AP shows that DOC is consumed when P-acquiring enzymes are produced. The positive correlation between <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="cc9fcbc3808c845ed7a1b90adb40bd46"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-15-4481-2018-ie00002.svg" width="24pt" height="15pt" src="bg-15-4481-2018-ie00002.png"/></svg:svg></span></span>–N and <span class="inline-formula"><i>β</i></span>G suggested the increased availability of N promoted the decomposition of C. More extracellular enzymes that degrade soil organic matter are produced when there is understory vegetation, which leads to losses of soil C. On the other hand, the soil C sink is maintained by increased inputs of C. We can therefore conclude that understory vegetation contributes to C sequestration in Chinese fir forests and suggest that understory should be maintained to sustain soil quality in subtropical Chinese fir plantations.</p>