Core Ideas Long‐term annual application of manure maintained the soil pH but inorganic fertilizer decreased it. Manure application increased soil organic carbon (SOC) and total nitrogen (TN). Higher manure rate helps in improving the water stable aggregates compared to inorganic fertilizer at 0‐ to 10‐cm depth. Manure enhances soil fertility and crop yield; however, an optimum rate of manure application is important to avoid any negative impacts to soils and the environment. This study was conducted to assess the long‐term impacts of manure and inorganic fertilizer rates on soil organic carbon (SOC), total nitrogen (TN), water stable aggregate (WSA), pH, and electrical conductivity (EC) under corn (Zea mays L.)‐soybean (Glycine max L.) rotation at Beresford (established in 2003) and Brookings (established in 2008), South Dakota (SD). Study treatments included low (LM), medium (MM) and high (HM) manure, medium (MF) and high (HF) inorganic fertilizer, and control (CK). Soil samples were extracted from four replicates at 0‐ to 10‐cm, 10‐ to 20‐cm, 20‐ to 30‐cm and 30‐ to 40‐cm depths from either site in 2015. Results showed that manure application maintained the soil pH at 0 to 10 cm depth; whereas, inorganic fertilizer decreased it compared to the control treatment at either site. The highest SOC concentrations at 0‐ to 10‐cm depth were observed under HM (38.3 g kg–1) as compared to that under MM (30.9 g kg–1), LM (27.6 g kg–1), MF application (24.0 g kg–1), HF (25.8 g kg–1) and CK (23.3 g kg–1). Furthermore, HM treatment significantly increased SOC for each depth increment from 0‐ to 40‐cm compared to the inorganic fertilizer at either site. A similar trend was observed for the TN but differences were not always significant. On an average, manure increased the EC (1.56 dS m–1) by 2.2 times compared to that of fertilizer (0.71 dS m–1) for 0‐ to 10‐cm depth. Similarly, manure significantly increased WSA by 7.2 and 5.6% compared to that of fertilizer at 0‐ to 10‐cm depth for Brookings and Beresford, respectively. Data from this study concluded that the long‐term annual application of manure improved selected soil properties compared to that of inorganic fertilizer.

Manure impacts labile pools of soil organic carbon (SOC) and nitrogen (N) which can influence soil microbial composition (MCC) and enzyme activities, and hence soil health. The present study was conducted to investigate the impacts of long-term dairy manure and inorganic fertilizers (INF) on soil carbon (C) as well as nitrogen (N) fractions, enzyme activities, and microbial community structure in different time horizons at planting (P), one month after planting (1MAP), and after harvesting (H) under corn ( Zea mays L.)-soybean ( Glycine max L.) rotation. Study treatments included three manure application rates (low, phosphorus-based recommended rate; medium, nitrogen-based recommended rate; and high, the double rate of medium nitrogen based recommended rate), two INF rates (medium only nitrogen additions; and high nitrogen, phosphorus, potassium, zinc, and sulfur additions) and a control (no application of manure and/or inorganic fertilizer). In comparison to the INF, the dairy manure not only significantly increased chemical fractions of C and N but also impacted the enzyme activities. Average urease activity after manure was applied was shown to be 26.8% higher than it was with INF applied at planting. The β-Glucosidase activity was 6 and 14% higher with manure than it was with INF at 1MAP and harvesting, respectively. The cold-water extractable nitrogen (CWEN) was enhanced with high manure rate at all timings of sampling compared to the high fertilizer rate (53%), and CK (90%). Principal component analysis indicated that MCC under manure differed from those under the INF treatments. The total bacteria/total fungi ratio at planting was increased with the INF compared to the manure addition. Pearson’s correlation analysis showed that CWEC, CWEN, and enzyme activities especially β-Glucosidase activity were the key determinants of MCC. Data from this study showed that, compared to inorganic fertilizers, manure can be beneficial in enhancing soil health indicators.

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