Efficiency of mineral and organic fertilizers across two continents

To mitigate climate change, greenhouse gas emissions from the agricultural sector need to decrease. In this light, increasing agronomic use efficiency of nitrogen (N) application (i.e., additional grain yield per kg of N applied) is a promising avenue to attain similar yields with less inputs in regions such as Europe (with high N inputs). In contrast, on the African continent, N inputs need to increase to raise yields, which may contribute to improved food security and prevent land use change. In such case, increasing agronomic N use efficiency (N-AE) and simultaneously increasing N inputs can also be a mitigation strategy by decreasing losses to the environment and improving profitability. In both contexts, it is relevant to understand how much N-AE can be increased in a certain location, compared to the current status, and which N source (organic and/or mineral fertilizer) will be most efficient. In this working paper we present ongoing work on N benchmarking from the crop nutrient gap project (full name: Bringing Climate Smart Agriculture practices to scale: assessing their contributions to narrow nutrient and yield gaps). First, we compare current observed N-AE to the values they could potentially reach under optimal agronomic management. For this, we propose a new benchmarking method based on recent insights on the shape of N response curves and introduce the related ‘degree of good agronomy’. Second, we compare the performance of mineral versus organic fertilizers for cereal cultivation on two continents (Europe and sub-Saharan Africa) based on large number of field experiments. Finally, we assess whether and how N-AE of mineral N fertilizer can be improved when combined with organic amendments. Preliminary findings show that the proposed benchmarking method can work but relies on availability of data on soil N supply, potential yield and attainable yields. Currently, this information is sparsely available which might be a barrier for uptake of the method. We show that N supplied by mineral fertilizers is taken up more efficiently than from organic sources, with variation depending on the type of organic amendment. Variation was larger for sites in Africa than Europe, which makes targeted fertilizer strategies less straightforward. Based on European experimental data, we show that organic amendments do not increase the N-AE of mineral fertilizer N application, most likely due to the increased total N availability. In future research, we hope to improve the data requirements for the proposed benchmarking method, assess drivers of variation for nitrogen fertilizer replacement values of organic amendments and disentangle effects of organic amendments on the efficiency of mineral fertilizer N use, while extending our analysis to tropical regions

Circularity indicators and their relation with nutrient use efficiency in agriculture and food systems

Circular resource use in agriculture and food systems could play an important role when aiming for sufficient food output with limited environmental impact and resource depletion. Circularity, however, is not a goal in itself. With respect to nutrient use and emissions, agricultural system sustainability is currently commonly assessed by nutrient output/input ratio (O/I, nutrient use efficiency) or surplus per ha (I–O). OBJECTIVE: Our aim is to assess how these sustainability indicators are related to nutrient cycling. METHODS: Starting from basic circularity concepts, a set of equations (frame) is presented that relates nitrogen (N) and phosphorus (P) cycling to food product output, or to food use by human consumers. Circularity indicators express how many times a nutrient input cohort completes a full cycle (CyCt), or passes through the system's top trophic compartment (UseCt). Examples of such compartments are the crop (arable systems), the herd (livestock farms), and the human population (regional food systems). UseCt governs export in useful product. The frame allows to predict equilibrium O/I from system properties, and to attribute parts of O/I to direct (linear) and cycled flow. CyCtR quantifies how many cycles could be completed by nutrients in absence of product export. CyCtR allows to assess the efficacy of returning waste from exported products. Above indicators are compared against Finn cycling index and Figge circularity index, more commonly used in ecological and industrial research respectively. All indicators are calculated for systems of increasing complexity: (i) a UK wheat field, (ii) a Dutch dairy farm, and (iii) the Flanders regional food system. Their responses to changes in system properties are analysed for examples ii and iii. RESULTS AND CONCLUSIONS: Nutrient flows in UK arable field and Flanders are almost linear. In UK arable field, O/I equals 0.74 (N) and 0.66 (P), with small contributions from cycled flow (9% for N, 5% for P). In Flanders, cycled flow constitutes only 2% of total N and P flows that reach the human consumer in Flanders. The dairy farm shows largest contributions of cycled flow: 35% (N) and 60% (P) of O/I comes from cycled flow, but O/I itself is only 0.28 (N) and 0.72 (P). SIGNIFICANCE: The presented frame allows to assess the impacts of system changes on productivity, nutrient cycling, resource use and nutrient emissions. This is useful for ex-ante assessment of measures that reduce nutrient losses from the system or increase the retrieval of external waste flows

European survey shows poor association between soil organic matter and crop yields

A number of policies proposed to increase soil organic matter (SOM) content in agricultural land as a carbon sink and to enhance soil fertility. Relations between SOM content and crop yields however remain uncertain. In a recent farm survey across six European countries, farmers reported both their crop yields and their SOM content. For four widely grown crops (wheat, grain maize, sugar beet and potato), correlations were explored between reported crop yields and SOM content (N = 1264). To explain observed variability, climate, soil texture, slope, tillage intensity, fertilisation and irrigation were added as co-variables in a linear regression model. No consistent correlations were observed for any of the crop types. For wheat, a significant positive correlation (p < 0.05) was observed between SOM and crop yields in the Continental climate, with yields being on average 263 ± 4 (95% CI) kg ha higher on soils with one percentage point more SOM. In the Atlantic climate, a significant negative correlation was observed for wheat, with yields being on average 75 ± 2 (95%CI) kg ha lower on soils with one percentage point more SOM (p < 0.05). For sugar beet, a significant positive correlation (p < 0.05) between SOM and crop yields was suggested for all climate zones, but this depended on a number of relatively low yield observations. For potatoes and maize, no significant correlations were observed between SOM content and crop yields. These findings indicate the need for a diversified strategy across soil types, crops and climates when seeking farmers’ support to increase SOM.Data collection for this publication was financially supported by the European Commission under the CATCH-C project (Grant Agreement N° 289782) within the 7th Framework Programme for Research, Technological Development and Demonstration. Its content does not represent the official position of the EC and is entirely the responsibility of the authors

European survey shows poor association between soil organic matter and crop yields

A number of policies proposed to increase soil organic matter (SOM) content in agricultural land as a carbon sink and to enhance soil fertility. Relations between SOM content and crop yields however remain uncertain. In a recent farm survey across six European countries, farmers reported both their crop yields and their SOM content. For four widely grown crops (wheat, grain maize, sugar beet and potato), correlations were explored between reported crop yields and SOM content (N = 1264). To explain observed variability, climate, soil texture, slope, tillage intensity, fertilisation and irrigation were added as co-variables in a linear regression model. No consistent correlations were observed for any of the crop types. For wheat, a significant positive correlation (p < 0.05) was observed between SOM and crop yields in the Continental climate, with yields being on average 263 ± 4 (95% CI) kg ha−1 higher on soils with one percentage point more SOM. In the Atlantic climate, a significant negative correlation was observed for wheat, with yields being on average 75 ± 2 (95%CI) kg ha−1 lower on soils with one percentage point more SOM (p < 0.05). For sugar beet, a significant positive correlation (p < 0.05) between SOM and crop yields was suggested for all climate zones, but this depended on a number of relatively low yield observations. For potatoes and maize, no significant correlations were observed between SOM content and crop yields. These findings indicate the need for a diversified strategy across soil types, crops and climates when seeking farmers’ support to increase SOM

The legacy effect of synthetic N fertiliser

Cumulative crop recovery of synthetic fertiliser nitrogen (N) over several cropping seasons (legacy effect) generally receives limited attention. The increment in crop N uptake after the first-season uptake from fertiliser can be expressed as a fraction (∆RE) of the annual N application rate. This study aims to quantify ∆RE using data from nine long-term experiments (LTEs). As such, ∆RE is the difference between first season (RE1st) and long-term (RELT) recovery of synthetic fertiliser N. In this study, RE1st was assessed either by the 15N isotope method or by a zero-N subplot freshly superimposed on a long-term fertilised LTE treatment plot. RELT was calculated by comparing N uptake in the total aboveground crop biomass between a long-term fertilised and long-term control (zero-N) treatment. Using a mixed linear effect model, the effects of climate, crop type, experiment duration, average N rate, and soil clay content on ∆RE were evaluated. Because the experimental setup required for the calculation of ∆RE is relatively rare, only nine suitable LTEs were found. Across these nine LTEs in Europe and North America, the mean ∆RE was 24.4% (±12.0%, 95% CI) of annual N application, with higher values for winter wheat than for maize. This result shows that fertiliser-N retained in the soil and stubble may contribute substantially to crop N uptake in subsequent years. Our results suggest that an initial recovery of 43.8% (±11%, 95% CI) of N application may increase to around 66.0% (±15%, 95% CI) on average over time. Furthermore, we found that ∆RE was not clearly related to long-term changes in topsoil total N stock. Our findings show that the—often used—first-year recovery of synthetic fertiliser N application does not express the full effect of fertiliser application on crop nutrition. The fertiliser contribution to soil N supply should be accounted for when exploring future scenarios on N cycling, including crop N requirements and N balance schemes. Highlights: Nine long-term cereal experiments in Europe and USA were analysed for long-term crop N recovery of synthetic N fertiliser. On average, and with application rates between 34 and 269 kg N/ha, crop N recovery increased from 43.8% in the first season to 66.0% in the long term. Delta recovery was larger for winter wheat than maize. Observed increases in crop N uptake were not explained by proportionate increases in topsoil total N stock