Organic Carbon Linkage with Soil Colloidal Phosphorus at Regional and Field Scales: Insights from Size Fractionation of Fine Particles

Abstract

Nano and colloidal particles (1–1000 nm) play important roles in phosphorus (P) migration and loss from agricultural soils; however, little is known about their relative distribution in arable crop soils under varying agricultural geolandscapes at the regional scale. Surface soils (0–20 cm depth) were collected from 15 agricultural fields, including two sites with different carbon input strategies, in Zhejiang Province, China, and water-dispersible nanocolloids (0.6–25 nm), fine colloids (25–160 nm), and medium colloids (160–500 nm) were separated and analyzed using the asymmetrical flow field flow fractionation technique. Three levels of fine-colloidal P content (3583–6142, 859–2612, and 514–653 μg kg–1) were identified at the regional scale. The nanocolloidal fraction correlated with organic carbon (Corg) and calcium (Ca), and the fine colloidal fraction with Corg, silicon (Si), aluminum (Al), and iron (Fe). Significant linear relationships existed between colloidal P and Corg, Si, Al, Fe, and Ca and for nanocolloidal P with Ca. The organic carbon controlled colloidal P saturation, which in turn affected the P carrier ability of colloids. Field-scale organic carbon inputs did not change the overall morphological trends in size fractions of water-dispersible colloids. However, they significantly affected the peak concentration in each of the nano-, fine-, and medium-colloidal P fractions. Application of chemical fertilizer with carbon-based solid manure and/or modified biochar reduced the soil nano-, fine-, and medium-colloidal P content by 30–40%; however,the application of chemical fertilizer with biogas slurry boosted colloidal P formation. This study provides a deep and novel understanding of the forms and composition of colloidal P in agricultural soils and highlights their spatial regulation by soil characteristics and carbon inputs.

Implications of Free and Occluded Fine Colloids for Organic Matter Preservation in Arable Soils

“…High abundances of C org and Ca in the first size fraction (0.66–20 nm) of the free and occluded colloids (Figure a,b) were in accordance with other studies on the nanoparticle fraction of soil colloids. , A positive correlation between C org and Ca was also found ( r = 0.84, P < 0.01, Figure S7). The Ca along with Fe and Al in this fraction can be present as cations and interact with OM through O-containing ligands such as carboxylic and phenolic functional groups forming organometal complexes. ,, A minor proportion (4–7%) of the total colloidal C org was present in this fraction, similar to a recent study, in which the C org in the nanoparticle fraction (0.66–25 nm) of water-dispersible colloids (WDC) extracted from 10 agricultural soils (C content: 3–15 mg g –1 soil) accounted for 6–15% of the colloidal C org < 500 nm . In contrast, 32–90% of the WDC-C org was reported in the nanoparticle fraction (0.6–25 nm) from the organic layer of calcareous forest soils (C content: 73–467 mg g –1 soil) .…”

“…Three size fractions were found for the free and occluded colloids similar to other natural colloids in agricultural and forest soils ,, (Figure ). High abundances of C org and Ca in the first size fraction (0.66–20 nm) of the free and occluded colloids (Figure a,b) were in accordance with other studies on the nanoparticle fraction of soil colloids. , A positive correlation between C org and Ca was also found ( r = 0.84, P < 0.01, Figure S7).…”

“…High abundances of C org and Ca in the first size fraction (0.66−20 nm) of the free and occluded colloids (Figure 2a,b) were in accordance with other studies on the nanoparticle fraction of soil colloids. 11,15 A positive correlation between C org and Ca was also found (r = 0.84, P < 0.01, Figure S7). The Ca along with Fe and Al in this fraction can be present as cations and interact with OM through O-containing ligands such as carboxylic and phenolic functional groups forming organometal complexes.…”

“…Considering that the surface reactivity of mineral particles varies as a function of particle size, the colloidal fraction <220 nm (hereafter referred to as “fine colloids”) in soils, where pedogenic (oxyhydr)­oxides from Fe, Al, Mn, and different layer silicates have the highest shares, can be the most reactive and efficient in terms of SOM bonding. Associations between mineral colloids and SOM have been commonly found in soils under different management, e.g., arable, paddy, , forest, , and wetland soils . Among these studies, C org concentrations in colloidal organo-mineral associations and their subsized fractions were relatively well-recorded, − but their OM compositions were scarcely studied .…”

“…Associations between mineral colloids and SOM have been commonly found in soils under different management, e.g., arable, paddy, , forest, , and wetland soils . Among these studies, C org concentrations in colloidal organo-mineral associations and their subsized fractions were relatively well-recorded, − but their OM compositions were scarcely studied . In larger size ranges, the concentration and composition of SOM associated with particles normally differ from soil size fractions (e.g., sand, silt, and clay).…”