Priming of Soil Carbon Decomposition in Two Inner Mongolia Grassland Soils following Sheep Dung Addition: A Study Using ¹³C Natural Abundance Approach

<div><p>To investigate the effect of sheep dung on soil carbon (C) sequestration, a 152 days incubation experiment was conducted with soils from two different Inner Mongolian grasslands, <i>i.e.</i> a <i>Leymus chinensis</i> dominated grassland representing the climax community (2.1% organic matter content) and a heavily degraded <i>Artemisia frigida</i> dominated community (1.3% organic matter content). Dung was collected from sheep either fed on <i>L. chinensis</i> (C₃ plant with δ¹³C = −26.8‰; dung δ¹³C = −26.2‰) or <i>Cleistogenes squarrosa</i> (C₄ plant with δ¹³C = −14.6‰; dung δ¹³C = −15.7‰). Fresh C₃ and C₄ sheep dung was mixed with the two grassland soils and incubated under controlled conditions for analysis of ¹³C-CO₂ emissions. Soil samples were taken at days 17, 43, 86, 127 and 152 after sheep dung addition to detect the δ¹³C signal in soil and dung components. Analysis revealed that 16.9% and 16.6% of the sheep dung C had decomposed, of which 3.5% and 2.8% was sequestrated in the soils of <i>L. chinensis</i> and <i>A. frigida</i> grasslands, respectively, while the remaining decomposed sheep dung was emitted as CO₂. The cumulative amounts of C respired from dung treated soils during 152 days were 7–8 times higher than in the un-amended controls. In both grassland soils, ca. 60% of the evolved CO₂ originated from the decomposing sheep dung and 40% from the native soil C. Priming effects of soil C decomposition were observed in both soils, <i>i.e.</i> 1.4 g and 1.6 g additional soil C kg⁻¹ dry soil had been emitted as CO₂ for the <i>L. chinensis</i> and <i>A. frigida</i> soils, respectively. Hence, the net C losses from <i>L. chinensis</i> and <i>A. frigida</i> soils were 0.6 g and 0.9 g C kg⁻¹ soil, which was 2.6% and 7.0% of the total C in <i>L. chinensis</i> and <i>A. frigida</i> grasslands soils, respectively. Our results suggest that grazing of degraded Inner Mongolian pastures may cause a net soil C loss due to the positive priming effect, thereby accelerating soil deterioration.</p></div

Temporal dynamics of δ¹³C (‰ vs VPDB) signatures in CO₂ emitted during 152 days following sheep dung addition to <i>L. chinensis</i> and <i>A. frigida</i> soil (n = 3).

<p>Temporal dynamics of δ¹³C (‰ vs VPDB) signatures in CO₂ emitted during 152 days following sheep dung addition to <i>L. chinensis</i> and <i>A. frigida</i> soil (n = 3).</p

The relative contribution of dung-derived CO₂-C and soil-derived CO₂-C during 152 days incubation calculated from the δ¹³ C signature of CO₂ after sheep dung addition (n = 3).

<p>The relative contribution of dung-derived CO₂-C and soil-derived CO₂-C during 152 days incubation calculated from the δ¹³ C signature of CO₂ after sheep dung addition (n = 3).</p

Temporal dynamics of CO₂ fluxes (mg C kg^–1 day^–1) (A and B), and cumulative CO₂-C loss (mg C kg^–1 day^–1) (C and D) during 152 days of incubation of <i>L. chinensis</i> and <i>A. frigida</i> soils amended with C₃ and C₄ dung.

<p>Values are the mean (n = 3) ± SE (bars).</p

Characteristics of sheep dung used in the incubation study. Dung was collected from sheep either fed on L. <i>chinensis</i> (C₃ dung) or C. <i>squarrosa</i> (C₄ dung).

a<p>The content based on 60g fresh dung portions as applied in the experiment.</p>b<p>Numbers are mean ± SE of n = 3 replicates.</p

Primed CO₂ emission (mg C kg⁻¹ day⁻¹) during 152 days of incubation from sheep dung amended <i>L. chinensis</i> and <i>A. frigida</i> soils.

<p>The numbers indicate soil C derived CO₂ emitted in excess to the control soil (n = 3).</p

The dynamics of δ¹³C (Mean ± SE) in control soils and in soil treated with C₃ and C₄ dung, and the percent of dung-derived C incorporated in the soil in relation to soil C (D) and applied dung C (P).

<p>Data are shown for each sampling during the 152 days incubation.</p><p>Different superscript letters represent statistical significance at <i>P</i><0.05 at the same sampling time among treatments.</p

Fates of soil and sheep dung carbon after 152

a<p>Net soil C loss was given by the value of sheep dung C sequestrated in the soil during 152 days subtracted from the primed soil CO₂.</p>b<p>Soil C loss (%) is the percentage total CO₂-C loss compared to soil total organic C content.</p

Comparison of amino acid sequences of domains highly conserved in the polyproteins encoded by ORF3 of badnaviruses and a tungrovirus.

<p>The virus names and the positions of starting amino acid are indicated before each sequence. Identical (*) and conserved (:) amino acids are marked.</p

The silencing hot spots of TaBCHV-1(A) and TaBCHV-2(B) genomes and the predicated secondary structure around the vsRNA at 6432 nt (C).

<p>The start and stop positions of the vsRNA₆₄₃₂ are marked by bold arrows.</p