Effects of nutrient fertilization on root decomposition and carbon accumulation in intensively managed grassland soils

Abstract

Our knowledge of root mass decomposition processes has greatly benefited from studies carried out in natural and semi-natural ecosystems. It is less known, however, how root mass loss might occur in intensively managed ecosystems, such as agricultural grasslands, which receive chronic additions of nitrogen (N) fertilizers. Key questions remain about (1) how repeated N applications either in organic or inorganic forms might influence fine root mass decomposition, and (2) what implications root mass loss might have for soil carbon (C) storage. Here, we use a long-term grassland experiment established in 1970 at Hillsborough, Northern Ireland (UK), to address how repeated additions of organic (i.e., liquid manures) and inorganic (i.e., NPK) fertilizers over >45 yr might influence fine root mass decomposition and how. We found that after a 10-month incubation period under field conditions, highest root mass loss (~43%) was associated with highest additions of organic slurries, whereas lowest root mass loss (~29%) occurred in inorganically N-fertilized and unfertilized soils. Root decomposition was significantly and negatively related to root C:N ratios and fungal-to-bacterial (F:B) ratios. Root decomposition, however, was not related to changes in soil C stocks or rates of soil C accumulation. N applications either maintained or increased (up to three times) soil C accumulation compared to unfertilized soils after five decades of intensive management. Our findings suggest that in the short-term organic fertilization can enhance root detritus decomposition (thus promoting N release in soils for plant uptake) while contributing in the long-term to soil C accumulation either through extra C inputs from manures or through N-induced effects on microbial activity or both. Further studies need to address how this dual effect of organic fertilization could contribute making grassland intensification more sustainable by reducing the use of artificial NPK fertilizers.

Effects of Legume–Grass Ratio on C and Nutrients of Root and Soil in Common Vetch–Oat Mixture under Fertilization

“…Root associated C is one of the main contributors to soil C storage [44]. Returning organic matter through roots is closely related to soil nutrients [18]. In this study, N R concentration was significantly positively correlated with N S content, showing the close and positive interaction between plant and soil; this is consistent with the study by Ma et al (2015) [45].…”

“…Additionally, lower nutrient use efficiency also implies there is relatively adequate N and P availability in the mixtures. There is a significant negative correlation between root decomposition and C R concentration [18]. It is generally accepted that lower nutrient use efficiency, that is lower C:N R and C:P R , results in higher decomposition rate and soil nutrient cycling rate [29].…”

“…In addition, root chemistry seems to be one of the main factors controlling root litter decomposition [17]. The decomposition rate is significantly negatively correlated to root C:N in natural and semi-natural ecosystems [18]. Therefore, it is very important to deeply understand the relationship of C and nutrients between root and soil; however, few studies have been carried out to uncover how mixing ratio affects root C and nutrients and consequently affects soil nutritional status.…”

Fine root decomposition in forest ecosystems: an ecological perspective

“…Additionally, by altering plant histochemistry, the effect of soil nutrient availability on decomposition is more likely to indirectly (rather than directly) affect root decomposition rate (Jiang and Liang, 2022). Fertilizers applied only for a short periods can boost root detritus decomposition (thereby encouraging N release in soils for plant uptake) and contribute to long-term soil C accumulation through either additional C inputs from manures or N-induced effects on microbial activity (Fornara et al, 2020).…”

Effects of land use and climate on carbon and nitrogen pool partitioning in European mountain grasslands

“…Conversely, decreasing management intensity and abandonment of grasslands generally leads to decreased productivity and C and N turnover (Chang et al, 2021;Leifeld et al, 2015). However, there are still large uncertainties as to the consequence of altered grassland management and abandonment on the size of the different above-and below-ground C and N pools (Fornara et al, 2020).…”