Artificial root exudates and soil organic carbon mineralization in a degraded sandy grassland in northern China

Abstract

Plant root exudates contain various organic and inorganic components that include glucose, citric and oxalic acid. These components affect rhizosphere microbial and microfaunal activities, but the mechanisms are not fully known. Studies concerned from degraded grassland ecosystems with low soil carbon (C) contents are rare, in spite of the global distribution of grasslands in need of restoration. All these have a high potential for carbon sequestration, with a reduced carbon content due to overutilization. An exudate component that rapidly decomposes will increase soil respiration and CO 2 emission, while a component that reduces decomposition of native soil carbon can reduce CO 2 emission and actually help sequestering carbon in soil. Therefore, to investigate root exudate effects on rhizosphere activity, citric acid, glucose and oxalic acid (0.6 g C/kg dry soil) were added to soils from three biotopes (grassland, fixed dune and mobile dune) located in Naiman, Horqin Sandy Land, Inner Mongolia, China) and subjected to a 24-day incubation experiment together with a control. The soils were also analyzed for general soil properties. The results show that total respiration without exudate addition was highest in grassland soil, intermediate in fixed dune and lowest in mobile dune soil. However, the proportion of native soil carbon mineralized was highest in mobile dune soil, reflecting the low C/N ratio found there. The exudate effects on CO 2-C emissions and other variables differed somewhat between biotopes, but total respiration (including that from the added substrates) was significantly increased in all combinations compared with the control, except for oxalic acid addition to mobile dune soil, which reduced CO 2-C emissions from native soil carbon. A small but statistically significant increase in pH by the exudate additions in grassland and fixed dune soil was observed, but there was a major decrease from acid additions to mobile dune soil. In contrast, electrical conductivity decreased in grassland and fixed dune soil and increased in mobile dune. Thus, discrete components of root exudates affected soil environmental conditions differently, and responses to root exudates in soils with low carbon contents can differ from those in normal soils. The results indicate a potential for, e.g., acid root exudates to decrease decomposition rate of soil organic matter in low carbon soils, which is of interest for both soil restoration and carbon sequestration.

Do warming-induced changes in quantity and stoichiometry of root exudation promote soil N transformations via stimulation of soil nitrifiers, denitrifiers and ammonifiers?

“…Considering enhanced root-derived C fluxes and C: N ratio of root exudates along with the increased soil N transformation rates under experimental warming in this study, certain association may exist between the root exudation and soil N transformations. Some previous studies showed that warming might alter soil physicochemical properties (e.g., soil pH) by stimulating root exudation, thereby influencing the soil N availability [15,16], as some components of root exudates such as organic acid would decrease soil pH [16]. However, our results showed that the soil pH was not significantly influenced by warming (Table 1), which may be a result of the pH buffering capacity of soil [73].…”

“…Many studies have addressed how climate warming affected soil N transformations, from which three possible channels could be briefly summarized: (1) warming directly stimulates SOM decomposition and N transformations [9e11]; (2) warming indirectly affect soil N transformations by changing the soil microbial activity or the structure of microbial community [12e14]; (3) warming alters soil physico-chemical properties (e.g., soil pH) by stimulating root exudation, thereby influencing the soil N availability [15,16].…”

Species composition and diversity, and carbon stock in a dune ecosystem in the Horqin Sandy Land of northern China

“…On the contrary, the percentage of belowground carbon in the interdune lowlands of active dunes contributed the highest among the different interdune lowlands: the belowground carbon proportion was significantly higher in the interdune lowlands of active dunes than in the interdune lowlands of stabilized dunes or interdune lowlands of semi-stabilized dunes. Luo et al (2014) found that discrete components of root exudates affected soil physical-chemical conditions differently, and responses to root exudates in soils with low carbon contents can differ from those in normal soils. Their results indicated a potential for acid root exudates to decrease decomposition rate of soil organic matter in low carbon soils, which is of interest for both soil restoration and carbon sequestration.…”

Root Exudates Induce Soil Macroaggregation Facilitated by Fungi in Subsoil

“…By applying artificial root exudate mixtures as a surrogate for soluble rhizodeposits, their effects on soil processes can be examined. Most of the previous studies applying root exudates have focused mainly on biological parameters, such as soil respiration and microbial community structure and activity (Marx et al, 2010;Drake et al, 2013;Luo et al, 2014), and not on soil aggregation. Moreover, only few studies have investigated the effects of labile C additions to different soil depths so far, and if so, they were restricted to the determination of priming effects.…”