Nitrogen leaching and groundwater N contamination risk in saffron/wheat intercropping under different irrigation and soil fertilizers regimes

**Background:** Excessive nitrogen fertilizer use and improper irrigation in agriculture have increased nitrate leaching, threatening groundwater quality globally. In semi-arid regions like Iran, over-irrigation exacerbates this problem. Saffron (Crocus sativus L.) is Iran's most valuable spice crop, with Iran producing 90% of the world's saffron. Saffron is typically planted in rows with 0.25–0.35 m spacing, leaving inter-row spaces that could be utilized for intercropping. This study hypothesized that winter wheat intercropped in saffron inter-rows could capture residual nitrogen and reduce nitrate leaching, while organic manure and deficit irrigation could further mitigate environmental risks. No prior study had examined nitrate leaching reduction in saffron fields through intercropping.

**Methods:** The four-year study (2013–2017) was conducted at Shiraz University Research Station, Iran (29°43' N, 52°35' E, 1810 m elevation) in a semi-arid climate with 387 mm mean annual precipitation. Forty-eight in-field lysimeters (100 cm diameter, 110 cm length) were used in a randomized complete block design with three replications. Treatments included two cropping systems (saffron monocropping vs. saffron-winter wheat intercropping), two nitrogen sources (fermented cow manure at 30 Mg ha⁻¹ vs. granular urea at 120 kg N ha⁻¹), and four irrigation regimes (40%, 60%, 80%, and 100% of standard crop evapotranspiration, ETc). Saffron corms (>8 g) were planted in three 30-cm rows at 15 Mg ha⁻¹ density. Winter wheat (Shiraz cultivar) was planted at 250 kg ha⁻¹ in inter-row spaces. Irrigation was calculated based on saffron water requirements only, with no extra water for wheat. Drainage water volume and nitrate concentration were measured after each irrigation event. Plant nitrogen and phosphorus concentrations were determined using Kjeldahl and ammonium-vanadate-molybdate methods. Various efficiency indices (NAE, NUtE, NUE, NHI, PHI, SNBI) were calculated.

**Key Results:** The 60% ETc irrigation regime was identified as optimal, producing the highest saffron and wheat nutrient uptake, nutrient harvest indices, acquisition and use efficiencies, and crop yields (saffron stigma: 3.427 kg ha⁻¹; corm: 10.308 Mg ha⁻¹; wheat grain: 2.946 Mg ha⁻¹) while achieving the lowest nitrogen loss (SNBI: 21.93 kg ha⁻¹). Manure application significantly outperformed urea: it reduced drained water by 12%, leachate nitrate concentration by 42% (1.618 vs. 2.777 mg N L⁻¹), total leached nitrogen by 50% (2.7856 vs. 5.5252 kg N ha⁻¹), and SNBI by 46% (28.12 vs. 52.05 kg ha⁻¹). Manure increased corm nitrogen concentration by 9% (1.22% vs. 1.12%), phosphorus by 8% (2.18 vs. 2.03 g kg⁻¹), and protein by 9% (7.621% vs. 6.994%). Saffron stigma yield was 25% higher with manure (3.641 vs. 2.895 kg ha⁻¹), corm yield 21% higher (10.321 vs. 8.517 Mg ha⁻¹), irrigation water productivity 20% higher, and corm nitrogen use efficiency 39% higher. Intercropping reduced drained water by 10% (147.52 vs. 162.92 mm), leachate nitrate concentration by 11% (2.074 vs. 2.321 mg N L⁻¹), seasonal leached nitrate by 23% (3.6142 vs. 4.6966 kg N ha⁻¹), and SNBI by 64% (21.06 vs. 59.11 kg ha⁻¹) compared to saffron monocropping. However, intercropping reduced saffron yield by 15% and corm yield by 19% due to nutrient competition. Power regression models (L = aD^b) were developed to predict cumulative leached nitrogen from cumulative drained water for all treatment combinations (R² = 95.5–99.6%).

**Clinical Implications:** This study provides strong evidence that saffron-wheat intercropping combined with organic manure and 60% ETc irrigation is an effective, sustainable system for reducing groundwater nitrate contamination risk in semi-arid saffron-producing regions. The 50% reduction in total leached nitrogen with manure and 23% reduction with intercropping represent substantial environmental benefits. The 64% lower SNBI in intercropping indicates dramatically reduced nitrogen losses to the environment through denitrification, volatilization, and other pathways. While intercropping reduced saffron yields by 15–19%, the additional wheat grain yield and 14% higher total economic water productivity (0.5905 vs. 0.5166 US$ m⁻³) compensate economically. The empirical regression models provide practical tools for farmers and regulators to predict nitrate leaching based on drainage volumes. These findings are particularly relevant for Iran, where 11% of potable groundwater samples in Shiraz exceed the 10 mg N L⁻¹ drinking water standard, and for other semi-arid regions facing similar water quality and scarcity challenges.