Response of soil organic carbon and water-stable aggregates to different biochar treatments including nitrogen fertilization

Recent studies show that biochar improves physical properties of soils and contributes to the carbon sequestration. In contrast to most other studies on biochar, the present study comprise a long-term field experiment with a special focus on the simultaneous impact of N-fertilizer to soil structure parameters and content of soil organic carbon (SOC) since SOC has been linked to improved aggregate stability. However, the question remains: how does the content of water-stable aggregates change with the content of organic matter? In this paper we investigate the effects of biochar alone and in a combination with N-fertilizer (i) on the content of water-stable macro- (WSAₘₐ) and micro-aggregates (WSAₘᵢ) as well as soil structure parameters; and (ii) on the contents of SOC and labile carbon (CL) in water-stable aggregates (WSA). A field experiment was conducted with different biochar application rates: B0 control (0 t ha⁻¹), B10 (10 t ha⁻¹) and B20 (20 t ha⁻¹) and 0 (no N), 1ˢᵗ and 2ⁿᵈ level of nitrogen fertilization. The doses of level 1 were calculated on required average crop production using the balance method. The level 2 included an application of additional 100% of N in 2014 and additional 50% of N in the years 2015–2016 on silty loam Haplic Luvisol at the study site located at Dolná Malanta (Slovakia). The effects were investigated after the growing season of spring barley, maize and spring wheat in 2014, 2015 and 2016, respectively. The results indicate that the B10N0 treatment significantly decreased the structure vulnerability by 25% compared to B0N0. Overall, the lower level of N combined with lower doses of biochar and the higher level of N showed positive effects on the average contents of higher classes of WSAₘₐ and other soil structure parameters. The content of SOC in WSA in all size classes and the content of CL in WSAₘₐ 3–1 mm significantly increased after applying 20 t ha–1 of biochar compared to B0N0. In the case of the B20N1 treatment, the content of SOC in WSAₘₐ within the size classes >5 mm (8%), 5–3 mm (19%), 3–2 mm (12%), 2–1 mm (16%), 1–0.5 mm (14%), 0.5–0.25 mm (9%) and WSAₘᵢ (12%) was higher than in B0N1. We also observed a considerably higher content of SOC in WSAₘₐ 5–0.5 mm and WSAₘᵢ with the B10N1 treatment as compared to B0N1. Doses of 20 t biochar ha⁻¹ combined with second level of N fertilization had significant effect on the increase of WSAₘₐ and WSAₘᵢ compared to the B0N2 treatment. A significant increase of CL in WSA was determined for size classes of 2–0.25 mm and WSAₘᵢ in the B20N2 treatment. Our findings showed that biochar might have beneficial effects on soil structure parameters, SOC, CL in WSA and carbon sequestration, depending on the applied amounts of biochar and nitrogen.