Effect of manure on soil organic carbon concentration in dryland soils: a small descriptive meta-analysis of 4 field studies, with a 7-study fertilizer-matched analysis

Background. Organic amendments are often proposed to raise soil organic carbon (SOC) in low-carbon dryland and saline or sodic soils. This small, pre-registered meta-analysis was planned for all organic amendments, but the usable evidence turned out to be almost entirely about manure. Methods. I searched Google Scholar, CORE and AGRIS (427 records). Full texts were sought for 31 priority records and assessed for 15. Eight field studies were included and seven had usable variance data (20 effect sizes). I used the log response ratio (lnRR) and a multilevel random-effects model (rma.mv, REML). Two sets were kept apart: manure against an unamended control, and amendment plus fertilizer against the same fertilizer alone. My planned stopping rule (10 usable studies) was not met, and 232 records carried forward after title screening (8 of them untitled) were never assessed at full text. Six deviations from the registration are listed in Table 1. Results. Manure against an unamended control (4 studies, 5 effect sizes) raised SOC concentration (0 to 20 cm) by 51.4% (z-based 95% CI 38.4 to 65.6; t-based 95% CI 30.9 to 75.1; I² = 83%). The fertilizer-matched set (7 studies, 15 effect sizes, mixed amendment types) gave +47.9% (z-based 95% CI 25.7 to 74.1; t-based 95% CI 20.7 to 81.4; I² = 99%). Leave-one-out estimates for the main set stayed between +48.5% and +55.2%. No subgroup tests were possible. Conclusion. Every effect size was positive, but the result is descriptive only. It rests on 4 studies, all 18 to 30-year experiments, mostly with high manure rates. It should not be read as a general effect of organic amendments.