Research Projects

Research Projects

Polysulphate, biological nitrogen fixation and yield

Prof. Michael Castellano, and Prof. Sotirios Archontoulis (Iowa State University)

Biological nitrogen fixation (BNF) is highly sensitive to deficiencies of phosphorus, potassium and sulfur. As BNF increases as a proportion of total N uptake, so does the demand for P, K, and S. Legumes that predominately rely on BNF rather than soil N generally have greater demand for P, K and S because they require these nutrients in nodules and aboveground biomass; moreover, the concentrations are higher in nodules (Qiao et al. 2007). A number of reports demonstrate that P, K and S limitations on BNF can reduce crop production (Keyser & Li 1992; Purcell et al. 2000; Scherer 2001; Devito & Sadras 2014). Moreover, in the Midwest US Corn Belt, BNF is high and thus requires large amounts of non-N nutrients: our measurements from 16 site-years across Iowa (Cordova et al. 2019) found that BNF (kg N ha-1) is positively linearly correlated with soybean yield (Mg grain ha-1) such that a typical yield of 4 Mg ha-1 requires >130 kg BNF ha-1 (y = 16.30x+67.41).

Deficiencies of P, K and S can limit BNF both directly and indirectly. Direct limitations reduce nodule mass, number and activity. In particular, there are close relationships between S supply and the nitrogenase and leghemoglobin contents of nodules (Scherer 2008). Indirect limitations occur when nutrient availability limits plant growth and, as a result, the plant does not invest carbohydrates and nutrients in nodule growth and activity.  Nodule mass is highly sensitive to nutrient deficiency – more sensitive than shoot mass: a meta-analysis found that critical concentrations of P, K and S for maximum yield are generally greater in nodules than shoots (Divito & Sadras 2014). This widespread result suggests that indirect limitation is the primary mode of nutrient limitation on BNF. Moreover, P, K and S limitation can reduce BNF and soil N uptake; as a result, ratios of N/P, N/K and N/S in shoots may not be the most sensitive indicators of nutrient limitation on BNF (Divito & Sadras 2014).  In addition to limiting BFN and crop production, S limitation can reduce seed protein and therefore seed quality. Sulfur deficiency can reduce protein concentration as well as the concentrations of S-containing amino acids cysteine and methionine.

In the Midwest US, prediction of S deficiency and crop response to applied S fertilizer remains challenging. Over the past two years, work at ISU has demonstrated that crops do respond to S, and particularly Polysulphate. However, the response is not consistent. Moreover, confirmed early-season S deficiency can eventually disappear with no effect on grain yield (see final report from previous research agreement). However, as Midwest soybean yields continue to increase, so will the demand for BNF and nutrients including S and K.

Over the last 30 years, Iowa soybean yields increased at a rate of 0.57 bushels per acre per year. Statewide average yields are now about 60 bushels per acre (4.04 Mg/ha). At the same time, S inputs from atmospheric deposition have drastically declined and are <30% of the amount of S harvested in grain. The role of S may be particularly important because S can reduce both BNF and N uptake from soil. And, high-yielding soybeans are well known to be limited by N and other nutrients.

In one of four trails that measured soybean yield response to S fertilizer trials in Iowa, we observed a significant effect of S fertilizer on grain yield. In this trial, leaf N concentration at the R3 growth stage was also higher in treatments receiving sulfate-based S fertilizers (5.07% N) than treatments receiving elemental S (4.98% N) or no S fertilizer (4.86% N). Using allometric equations to estimate root and total aboveground biomass, these data suggest that soybeans receiving the sulfate-based S fertilizers accumulated >30 kg N ha-1 more than the other treatments. A substantial portion of the N is likely due to greater BNF.