Effects of nitrogen and phosphorus addition on phytolith carbon sequestration in alpine grassland

Abstract

Alpine grassland ecosystem plays a vital role in regulating global carbon (C) cycle, being of great importance for mitigating global warming. Applying nitrogen (N) and phosphorus (P) fertilizers illustrates significant effects on alpine grassland service function in Qinghai-Tibetan Plateau (QTP), especially on grassland ecosystem C sink. Phytolith C sequestration, as an important C sink mechanism, has significant contribution to global long-term stable C sink. However, the effects of N and P addition on phytolith C sequestration in alpine grassland ecosystem of QTP are still unclear. We conducted a field experiment with N and P addition treatments in alpine grassland of QTP to analyze their effects on soil properties, plant biomass, phytolith contents, and the occluded C (PhytOC) contents in alpine grassland ecosystem. Experimental results showed that adding N fertilizers (72 kg ha⁻¹ yr⁻¹) significantly increased the grass phytolith content, with an increase of 29.5 ± 17.5 % compared to the control, and with a 12.9 ± 9.1 % increase with adding P fertilizers (36 kg ha⁻¹ yr⁻¹). In contrast, the co-addition of N and P increased the soil phytolith and PhytOC contents (0–10 cm) by 40.9 ± 14.4 % and 58.9 ± 1.6 %, respectively. Yet excessive N addition reduced the contents of soil phytolith and PhytOC due to soil acidification and nutrient imbalances. In addition, both soil organic C and soil available silicon had significant positive correlation with soil phytolith contents, while soil pH and ammonium N had significant negative correlation with soil phytolith contents. These findings reveal that adding N and P regulates the long-term stable C sequestration in alpine grassland ecosystem via promoting the accumulation of phytoliths and phytolith C storage. Our findings provide the main scientific reference for the management of alpine grassland ecosystem on the QTP.